We redid the database conversion. Instead of moving from 253 > 258, 258 > 3.3.2, 3.3.2 > 4.2.1. We now moved (database-wise) from 2.5.3. > 4.2.1. in 1 go. For now the issue seems to have disappeared. We will test some more, but I do expect that the issue has been resolved by this retry.