The quick fix is to disconnect your Client from the Server, then on the Client go to;
C:\Users\[UserID]\AppData\Roaming\J River\Media Center 23\Thumbnails
In there you will find multiple sub-directories with names that look like CLSIDs. i.e. A long list of characters inside {} brackets.
Check the size of each. The largest sub-directory is likely the Thumbnails for your Server based Library. Delete that sub-directory. MC will rebuild the thumbnails when you reconnect the Client to the Server Library.
The above assumes that the thumbnails are working on the MC Server. If they are doing the same thing there, then under "Options > Tree & View > Thumbnails" you could try "Build missing thumbnails" and if that doesn't work then "Erase all thumbnails" then "Build missing thumbnails", but that is pretty drastic. The problem shouldn't happen on the MC Server if you used the "Cover Art > Get From Internet" function. It can happen if you just put a new file of the correct name into the \Cover Art\Seasons\ sub-directory, as it takes MC a while to detect the change, and it only builds the thumbnail when it has detected the change. The Cover Art image file, and the thumbnail shown in most places in MC, are different entities.