VapourSynth preview no longer saves preferences

[sharkeylaser]

No, not using the portable version. In all cases, I've used the installer.
When I go Tools->vsViewer, I get an error message:

Failed to open the file J:/tmp/tempPreviewVapoursynthFile08_10_39_097.vpy.

I've never had a J: drive, nor have I set any paths to J: by mistake. My "workspace" drive is E:, so it should be E:. I also set the temp folder to E:\temp, so J:\tmp (without the "e") is very wrong.
As such, the only two lines of the script are:

import vapoursynth as vs
core = vs.core

Going to Edit->Settings, unchecking "Portable mode enabled", hitting "Apply" and "OK" doesn't seem to work; the checkbox state isn't saved and reverts as soon as I open the settings page again.

Edit: attached debug log from that just in case.