Fresh install of LTS version. 5.18.
As well as the known issue of not being able to change fonts, I've also realised that folder locations can't be changed in system settings.
Whether the 2 issues are related, I'm not quite sure yet. I've noticed that 5.18.1 as now been released.
Interesting to see if that resolves the issues.

Just to add to this.
User Folder locations can be changed by manually typing the path in the location box.
The problem seems to be that the "open file dialogue" box won't autofill the Folder name, as it used to in previous versions.