First of all: use the hosted web app https://app.plex.tv
Disable all web browser add-ins. Some of them can interfere quite heavily with regular operation of the web app.
Check your Plex database for damage
- activate debug logging (not ‘verbose’!)
- quit Plex Server
- wait 1 minute
- start Plex Server
- wait 5 minutes
- fetch log files and attach them here
Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database.
https://support.plex.tv/articles/201100678-repair-a-corrupt-database/
Delete the .bundle folders of those old agents from your plex plugins folder https://support.plex.tv/articles/201106098-how-do-i-find-the-plug-ins-folder/
Note that this will only remove the agents themselves. If you still have media which where “matched” using these old agents, you might have to perform either ‘Fix Match’ or the full Plex Dance to change their match.
Have you ever started Plex server “As Administrator” user?
This could have messed up the access permissions of some files and folders within your Plex data folder
Shut down plex server and try to reset them all, so that the regular Windows user account, under which plex server is running has full access permissions again.
Doing this with Windows’ own tools can be frustrating and/confusing, so here is a 3rd-party tool to streamline it: Resetting NTFS files permission in Windows – Graphical Utility – lallous' lab
As a last resort, you could try to reset all agent settings to their defaults, using this procedure: [HowTo] Reset metadata agent preferences to defaults and whack caches
If all of this was triggered by the missing or foreign-language movie posters – be advised that TheMovieDB recently deleted a whole lot of posters (for some unkown reason).
