This seems to be where the source error occurs. I can only suggest one last thing to try, and then we’ll have to hope one of the proper experts can add in a fix if it doesn’t work.
If you go to mysql and run that command manually, and then have a look at its results for clues on how to fix it…
bench mysql
and then in the mysql system,
update tabComment set comment = substr(comment, 6, locate(':', comment)-6) where comment_type in ('Assigned', 'Assignment Completed')