You may need to clear the cache from your Plex Media Server from time-to-time. While it isn't something you will need to do very often, if at all, it may help if a plugin, or agent is causing problems.
When Plex makes HTTP requests through a plugin, channel, or agent, it caches those requests on the server. These cached requests won't typically cause issues, but knowing the location of the cache directories can be useful.
Plugin, channel, and agent caches
The cache directory for the plugins, channels and agents is located in the Plex data directory on the server. The actual directory path would be:
[PLEX data directory]/Plug-in Support/Caches
Within the above directory there are subdirectories for each plugin, channel or agent. To delete the cache, you would just delete the directory associated to the item.
For example, if you would like to clear the cache for "The Move Database" agent, you would delete the following directory:
[PLEX data directory]/Plug-in Support/Caches/com.plexapp.agents.themoviedb
An alternative to deleting the directory, is to rename it or move it to a different directory. This way if something does happen, you can recover the directory.
Plex also manages a cache directory for media. This directory will contain files such as photo thumbnails when viewing photos from a client, and temporary files for transcoding media.
The location of the media cache directory is:
[PLEX data directory]/Cache
While the transcoding files are usually temporary - for the time it takes to stream the necessary transcoded file chunks - the photo thumbnails are persistent. The thumbnail cache speeds up the viewing of the photo library as each photo won't need to have its thumbnail created.
The only real issue with the photo cache is that it can get large if you have a lot of photos. Other than that, there isn't any real benefit to clearing out the photo cache on your Plex server.
If you do wish to clear out the cache, then you can find the photo cache in the following directory:
[PLEX data directory]/Cache/PhotoTranscoder
To clear the cache, simply delete all directories in the PhotoTranscoder directory. Keep in mind that there will be a delay when viewing the photos again as the thumbnails would need to recreated and then added back to the cache.
While it isn't recommended, or needed, you can also clear out the Cache directory. Doing so, though, can decrease the performance of Plex until the cache is rebuilt.