Thanks for the info – I already tried this out, with no effect.
I also had a look in the error.log and found out that there are several columns in the db missing.
e.g. daystarttime etc. Are this just PRO features / tables or should they be also present in the free version?
Then I searched for this problem and found a page that altered the database structure: https://reformatcode.com/code/php/dates-not-changing-in-ads-rotate-plugin-wordpress
Now I seem to have the right database structure, but still the dates keep changing to 01/01/1970.