I had an error when I upgraded from OJS 3.3.0.11 to 3.4.0.1. The custom blocks were no longer visible. This was because the locales (en_US) in the plugin_settings table were not converted. I could solve the problem by changing the locales manually (e.g. en_US → en).
Please check the status of this issue for a fix regarding your issue.
Don’t hesitate to add a follow-up comment if you encounter any problems after the above mentioned issue is closed, and the corrected migration is applied on your installation/database.
Don’t forget to keep a database backup before any migration attempt.