by Ken Task.
First and foremost ... it's never a good idea to hack/change core code unless you know what you are doing. Suggest that, from your description, that the only reason in knowing location of the file is to look at it ... ie, your curious - which is OK, just don't change.
It tells you in the error:
moodlecode/backup/util/ui/restore_ui_stage.class.php Line 157
The ONLY reason I shared that with you was for you to look to see if that line looked like the one I have on my server - which doesn't have any such issue.
Rather than beginning what could be a very lengthy discussion, let's just cut to the chase:
Turn on debugging
Site Admin menu -> Development -> Deugging
Set debug messages to Developer
Save changes
then try your restore process again. Are you dragging and dropping the .mbz file? or are you using other process to upload the .mbz file?
The output will display on your screen with, hopefully, more information.
Copy that info to NotePad. Come back to this forum, and respond pasting
the info from NotePad.
'spirit of sharing', Ken