We upgraded our Clarity from 12.1 to 13.3 going through proper upgrade path. There is one custom Object whose instances just fail to open in Edit View.
I can see the List view properly, however, on clicking the link to go inside the record, I get an error "Unable to Process Request - Server Error". It happens for all the records of that particular Object. I tried checking server logs, but nothing unusual appears there.
The same records were opening very easily in 12.1.