Is the read-only attribute set? Or have there been other advanced permissions changed so the file is read-only.
It's only the read only attribute. No special permissions.
Also, is this file on a remote file system?
This is on a local SSD.
It seems, that this might be related to version control.
Trying the steps in an unrelated directory works correctly, but doing the same in the directory where the sources of the current project reside, gives the error.
The project is using git.
Also a further observation:
Closing SE (even doing a windows restart) and reopening SE still shows the buffer as writable. Only closing the buffer and reopening it will update the state to read only.