I was about to give up on this - but gave it a new try.
I had been searching for one of the words from the error message but used the setting "exact match" in phpmyadmin - did a new search but with "one of the words".
It turned out that some "old" items was listed in "sys_datalog". I deleted these and everything is now working fine.
New question : Should "sys_datalog" be empty? - noted that mine has a little over 16.000 lines all with status "pending"