if something could not migrated then try to reimport the raw files (not the uasset files) to your project.
There is also sometimes a bug where some files are not correct migrated and cause errors(most times with blueprints).
Go into your log folder (in your project) and open the newest logfile.
Start at the bottom of the file and work from down to the top, there are the errors listed, backup and delete these files which causes errors.
Then start the editor without the error files.
If the editor is started you can copy the backups back in there while the editor is started.
The editor should instantly refresh the files while running and you can open and correct some errors (if its a blueprint then some nodes are not working anymore, you need to replace them).
Sometimes i was getting a error with some blueprints and they stop me from using the project again until i delete them and correct them then everything was working again.