step to reproduce:
1) upload a file to your private file area (eg example.jpg)
2) link it to a course
2.1) go to a course and add new file
2.2) in the content box click add, in file picker select private files, select the file, and choose "create an alias..." in the popup.
2.3) save and return to course
3) link it to another course (see step 2)
4) go to my private files
5) click add, upload a different file, save as the previous file name (eg. example.jpg), click overwrite.
(minor bug here is that the thumbnail doesn't refresh in file list)
6) click save changes

ERROR:
Coding error detected, it must be fixed by a programmer: PHP catchable fatal error

More information about this error

Debug info: Argument 1 passed to file_storage::update_references_to_storedfile() must be an instance of stored_file, null given, called in /var/www/clients/client1/web6/web/lib/filelib.php on line 835 and defined
Error code: codingerror
Stack trace:
line 397 of /lib/setuplib.php: coding_exception thrown
line 1823 of /lib/filestorage/file_storage.php: call to default_error_handler()
line 835 of /lib/filelib.php: call to file_storage->update_references_to_storedfile()
line 291 of /lib/filelib.php: call to file_save_draft_area_files()
line 64 of /user/files.php: call to file_postupdate_standard_filemanager()