Thank you once again George.

I'll check if our backup software is using SQL agent.

Nevertheless I did another test that might prove that the problem isn't the backup procedure but something else.

I've scheduled LS to stop everyday for one hour (normal hour in the afternoon when nothing's happening besides normal work). It still gave the Out of Sync error after resuming LS automatically.

Will test it again today, but I still don't understand what might be causing it.

Best Regards.