As per your other respondent - I had the same issue and was told that Oracle is still trying to check UNDO in the offlined tablespace. If the tablespace had been dropped then Oracle would not have bothered, making the assumption that the drop would not have taken place if there was still data in the UNDO tablespaces. So the solution is as posted if you feel confident to do so (I must admit I did not and ended up shrinking the old UNDO as small as possible and leaving it on-line but not used).
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