Backup to big. (Technics)
Beside the fact that we forgot to add new tables to the backup process over the time for several times, we (Micha and me) think the backup script is more or less rudimentary and old in the meaning of it's concept. At the end we recommend to use tools like phpMyAdmin for the backup.
That is the most important part of your message. I have to do backups with third-party tools.
Thank you also for your technical explanations. Let me just for the sake of completness, make the point I was wondering about more clearly:
I made backups over the years. Last year, entries log was 160MB (say) and very slowly increasing. This year, it was 164MB. Then I had troubles, and doubled the memory_limit. It worked one or two times with a backup size of 164,5MB or 165MB, then it stopped again working. Again i increased the memory_limit, now to three times of the original value, but it seems still not sufficient...
It looks like that a rather small increase of the database size does outperforme a memory_limit of the factor three.
But we can stop the discussion here, since I have to change my tooling anyway.