Index on sys_file_processedfile is too large
Since #54234 a SQL error in the Upgrade Wizard / Database Compare is left open:
ALTER TABLE sys_file_processedfile ADD KEY identifier (storage,identifier);
Specified key was too long; max key length is 1000 bytes
"identifier" is varchar(512) but on utf8 charset every character needs 3 bytes of space. So the index is "too large" (for MyISAM).
Solution would be to limit the key to the first 332 chars.