Regarding the actual problem of the DB error message I can't tell off the top of my head, so I'll focus on clean-up procedures.
I guess that the repository you're trying to copy has version control enabled (otherwise deleted objects would no longer exist in the repository and hence not in the backup file). Now one has to know that objects which are "deleted" from a versioned repository are not physically deleted; they are marked as "deleted", meaning they have a so-called "object status" of Deleted. When you use the repository manager, you can create a so-called repository query to search for such deleted objects; this can be handy in order to un-delete those objects.
Now "deleted" objects in a versioned repository exist as long as they are not physically removed from the repository. This can be done by "purging" deleted objects. Please consult the Repository Guide for PowerCenter for details how to do this.
You should purge all deleted versions from the original repository before trying to copy it via backup / restore.
If that was too much information, please ask for clarification; I know that sometimes I'm "slightly" wordy.
thank you very much for your response. It took some time to double/triple check some things. The actual problem was still the DB error which I was able to fix today. The problem was a small typo, so the wrong DB-Schema was used, which had fortunately/unfortunately (depends how you want so see it) different NLS-Settings. So https://kb.informatica.com/solution/23/pages/63/517427.aspx was still the correct solution. Or in my case, using the intended schema.
Nevertheless I'm going to purge the repository to get rid of those deleted objects. So thank you for the heads-up regarding the purging and the Repository Guide.
And no worries about too much information, I prefer it like this instead of the other way around.
Thank you for the assistance.