The "last edited by" turned out to be the only "visible" problem at that time.
The board has been fixed and updated (about a month ago IIRC).
How the board got into the state it was is unclear to me but the database had 4 or 5 sets of phpBB tables, each with a different prefix and not all complete, and with different collations (latin_swedish, utf_general_ci, and 2 more)
Also there was a mix of character sets within tables.
The 1970 date when editing a post without reason was due to
"
being the default value (in many tables).
The config_table had a number of fields which I couldn't place so there was some trial and error involved but it turned out that all those entries could be removed. Some extensions were outdated (some with no uptodate versions available so removed the redundant ones) and good 'ol tapatalk was there also, non functional (very old version so removed that completely).
I basically had to modify table by table, even field by field (default values, character sets) on a copy on localhost (using a fresh 3.3.3 board as an example) (so it was editing using the graphical phpMyAdmin features and then testing until I got the SQLs right and then used a fake database backup to execute the sqls on the server itself - I did have FTP access and a Founder account, but no phpMyAdmin access to the live server).
Then a couple of days of live testing just to make sure, and then updating the board to 3.3.4.
It's been running smoothly for about a month now.
I'm starting to believe the only way to kill a phpBB board is to delete it completely