I am having a similar problem. I've got three jobs with mail triggers. All three have a task to move attachments from incoming emails, then a task to delete the folder created by the mail trigger.
These worked until I upgraded from 6.5 (I think) to 7.0.7. I saw today that the folder delete task was erroring out, and discovered that the Base folder path had been erased out of the task configuration. I put the proper base folder back in,
E:\temp\VC-Docs\{TRIGGER(Active|LastTrigger|Mail.Result.Mail.MessageId)}
and tried again. Now I get an error like the one windmanager describes. If I show the filtered folders, I get, for example,
E:\temp\VC-Docs\1875
E:\temp\VC-Docs\1875\attachments
just like I would expect, but the folders do not actually get deleted. I can delete them manually with no errors.