File Backup issues an error

I'm essentially backing up my complete thunderbird e-mail folders every day onto a 2nd hardisk.
Since I updated to 6131 I get an error message that the backup could not finish.
Since my language settings are german I try to translate the messages I see but it might not be the message as it really would appear in english. Sorry for that.

Looking into the log I see that the backup itself should have been correct. Most of the conducted actions have a green tick.
It goes as far as "Backup-Archive validate" (green ok), "Backup-Archive consolidate" (green ok) but then, next line "open data stream failed" (not ok, red cross). Then, "update of the backup data base done..." (ok) and last "action ended with errors" (not ok, red cross).

Strange thing is when I look back into the logs there was a period of about a week back in october when no errors resulted. I started with the trial version in september and I now recognize the same errors at the beginning, then about a week when it was ok and from then on errors again.
I'm running Win7 64bit.

Does anybody see similar things?
Any suggestions?