When I try to create a new code template, I type in the name and click OK, but then I get a message saying "Count not create xxx", but no other details as to what's wrong - see screen shots attached. I'm using v15.0.3.2059 64bit on Windows 10.

You probably need administrator rights to write to this folder. In Preferences => Files => Directories you can supply a different location for your templates (and for your plugins etc.)

Thanks Patrick, but I am an administrator on this PC so I didn't expect any issues. Your solution is indeed a workaround, but I now find that I also cannot save any changes to the AutoReplace.txt file - getting Access Denied (see attached image), so this issue would seem to be across the board. I assumed that during the installation all relevant privileges would be granted to all relevant directories, but apparently not. Any idea how I might adjust those privileges?

OK I just resolved this issue (and the problem updating the AutoReplace.txt file) myself by giving ownership of the PLSQL Developer 15 directory (and all sub directories) to the Administrators group in Windows 10 (previously the directory was owned by SYSTEM) - this may help others with the same issues (if there are any!)