Okay, this has been solved. But this is the last straw for this stupid add-on managemt system which interacts with the file system.
In the future, I will make the add-on manager easier to use: You won't even have to worry about the filesystem. Add-ons will never be deleted unless you do it from FTP. So all you will get is a scroll pane full of add-ons which you can either enable or disable. That's the easiest I can make it.
Also, the add-on uploader will probably be removed. Somehow the web server stores the files in RAM although I am clearing the buffers properly and saving the uploaded add-ons to disk. If you want to upload an add-on, do it with FTP.
The new design will be based on the concept that filesystem-related stuff like uploading or deleting is to be done with FTP, and configuration stuff like disabling an add-on from the list or adding a new one to the list, is to be done with the c-panel.