We have found several files in Resources tool that can not be deleted at any time.
Tracking the issue, it seems that a NullPointerException is thrown when Kernel tries to store these resources in the deleted resources table.
NPE is not managed and deletion process is aborted, so these resources are permanently locked.
We have solved it with a simple catch block, though maybe Kernel experts would like to look deeper in the problem. Anyway, I attach our little patch.
Unfortunately, I can not replicate the steps to detect the issue. If you need more info, please ask it to me.