Hi Markus Berchtold,
This seems to be a bug only when you are in multilingual mode and for other content items. I was able to reproduce your scenario with the image libraries without issue - after renaming the first library I successfully created a new one with the old name. I don't see the fix for additional urls for multilingual mode in the release notes for SP1 though so I guess it will be fixed in the next service releases.
the Telerik team
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