If you are using a plex server version of 1.43.0.x (i.e. a beta version), ensure that you are using the very latest beta. If you don’t want to use beta versions, go back to the most recent release of the 1.42.x branch.
(I hope you didn’t enable “store track progress” in the properties of your music library?)
I can only guess what’s happening.
Are you using Plexamp on a mobile device? If so, is free storage space on this device rather low? Try reducing the cache size in the settings of Plexamp (Playback - Caching)
or free up more memory on the device by getting rid of other apps or their downloaded or collected data.
Or restrict it from streaming full quality files, if you have a lot of hires, lossless music. When using ear plugs, particularly when out & about, you’ll rarely notice the difference.
