Plex cannot show newly added files, but existing ones are OK

Thanks for the logs. The problem was immediately found.

May 15, 2020 10:10:26.230 [0x7f29acaec700] INFO - /var/packages/Plex Media Server/target/Plex Media Server
May 15, 2020 10:10:25.864 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FFA6" (28: No space left on device)
May 15, 2020 10:10:25.865 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FF50" (28: No space left on device)
May 15, 2020 10:10:25.887 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F619" (28: No space left on device)
May 15, 2020 10:10:25.888 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F753" (28: No space left on device)
May 15, 2020 10:10:25.945 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F1ED" (28: No space left on device)
May 15, 2020 10:10:25.958 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F1EC" (28: No space left on device)
May 15, 2020 10:10:25.959 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F5FD" (28: No space left on device)
May 15, 2020 10:10:25.960 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FD5B" (28: No space left on device)
May 15, 2020 10:10:26.014 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F77E" (28: No space left on device)
May 15, 2020 10:10:26.117 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F210" (28: No space left on device)
May 15, 2020 10:10:26.155 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FD4E" (28: No space left on device)
May 15, 2020 10:10:26.167 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F79F" (28: No space left on device)
May 15, 2020 10:10:26.183 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F800" (28: No space left on device)
May 15, 2020 10:10:26.192 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F1D7" (28: No space left on device)
May 15, 2020 10:10:26.204 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FC95" (28: No space left on device)
May 15, 2020 10:10:26.205 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F02C" (28: No space left on device)
May 15, 2020 10:10:26.213 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F4CA" (28: No space left on device)
May 15, 2020 10:10:26.230 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FEA9" (28: No space left on device)
May 15, 2020 10:10:26.243 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F980" (28: No space left on device)
May 15, 2020 10:10:26.273 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F2E9" (28: No space left on device)
May 15, 2020 10:10:26.281 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F10D" (28: No space left on device)
May 15, 2020 10:10:26.304 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F857" (28: No space left on device)
May 15, 2020 10:10:26.305 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FF35" (28: No space left on device)
May 15, 2020 10:10:26.322 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FE25" (28: No space left on device)
May 15, 2020 10:10:26.323 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FA09" (28: No space left on device)
May 15, 2020 10:10:26.342 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F56A" (28: No space left on device)
May 15, 2020 10:10:26.348 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FBD7" (28: No space left on device)
May 15, 2020 10:10:26.361 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FC24" (28: No space left on device)
May 15, 2020 10:10:26.369 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F7A2" (28: No space left on device)
May 15, 2020 10:10:26.377 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FD3D" (28: No space left on device)
May 15, 2020 10:10:26.420 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F792" (28: No space left on device)
May 15, 2020 10:10:26.430 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FFEB" (28: No space left on device)
May 15, 2020 10:10:26.437 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F913" (28: No space left on device)
May 15, 2020 10:10:27.052 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FA80" (28: No space left on device)
May 15, 2020 10:10:27.066 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F7D9" (28: No space left on device)

The automatic watch notifier list is full. Photos and Music will typically QUICKLY exhaust the default available space of 8192 folders…

This value needs to be increased.

May 15, 2020 10:10:26.230 [0x7f29acaec700] INFO - /var/packages/Plex Media Server/target/Plex Media Server
May 15, 2020 10:10:25.864 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FFA6" (28: No space left on device)
May 15, 2020 10:10:25.865 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/FF50" (28: No space left on device)
May 15, 2020 10:10:25.887 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F619" (28: No space left on device)
May 15, 2020 10:10:25.888 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F753" (28: No space left on device)
May 15, 2020 10:10:25.945 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F1ED" (28: No space left on device)
May 15, 2020 10:10:25.958 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F1EC" (28: No space left on device)
May 15, 2020 10:10:25.959 [0x7f295ae6b700] ERROR - [Notify] Failed to add watch for "/volume1/Backup/photos/M-pictures/Lightroom/Lightroom Catalog Previews.lrdata/F/F5FD" (28: No space left on device)

The procedure is herer.