Ok so its probably not my recent frequency scaling changes. I'm not sure then, have you tried any builds since 3.13 but before this month? A lot has changed over the last few years.
No, I haven't tried any of the previous builds. I have been using the Rockbox utility to pull down the stable and development versions. Is it just a case of unzipping these builds and then copy/paste onto the root of the clip? (deleting the previous version first).
I'll test a few and report back which is the latest working version.
EDIT: I have tested the latest builds of each Rev (2016-02-16, 12, 7 and 6) and all show a blank screen.