'Couldn't find required item at project path'

After you relinked the artwork, did the error message go away? Could you share a screenshot of the rigging window? Are you using any fancy things in photoshop? Not all features are supported. I don't recall that particular error before. 

There are different reasons that have come up in the forums. Bugs, software crashing half way through updating files on disk, network file system usage (locking does not work properly), disk full, etc. One thing you can try is the refresh button at the bottom of the scene. Click it, then alt-click it, the shift-click it... i always forget which, but one of them refreshes more internal caches which has fixed the occasional problem. I give 10% chance to fix the problem, but sometimes it does. If the files on disk got corrupted, i normally leave that to the Adobe folks who are often on and can repair things. I would keep a zip of the project as is - sometimes they investigate to see if they can track down the bug.