There is indeed a resolution – AND a procedure for upgrading from DSM 6 to DSM 7
DSM 7 is a complete rewrite and restructure from DSM 6 and Plex had to comply with those changes. (DSM 7 Plex is a complete rewrite)
We have our Synology FAQ
In there, of specific interest, which is predicated on whether or not “Plex/Library/Application Support/Plex Media Server” exists. is:
The process of migrating (converting DSM 6 Plex → DSM 7 Plex) takes time.
The speed is roughly 340 media items (movies, episodes, tracks, or photos) per minute.
There is extensive documentation on how to transition without losing data – provided the all steps are completed in order.
The recommendation is to read, understand what will be asked of you before starting. then start the process and allow DSM & Package Center to do their jobs.
AFTER PMS is migrated to DSM and the server is running, All the file permissions will need changing.