On a J3.10.6, we have try to upgrade from a very recent K3.1.6 to 4.0.2 with very little database (10 comments maybe ?).
After 20 min it stuck on Initializing Database Synchronization and the BO show a us a wonderfull 0 Class 'FH' not found
What is the next best move ? Of course we have backup and can reverse back but if you have some ideas to finish this nicely...