Some extra info on the problem.
The error text is the same regardless of which display I open. If I try to edit the display the values are displayed for a short time which I managed to catch a screengrab off, but not in the correct location, they are offset.
The displays refuse to save even when all objects are deleted.
The update log seems to suggest that these were updated
widget/feedtimestamp/feedtimestamp_render.js | 274 +++++++++++++++++++++++++++
widget/jgauge2/jgauge2_render.js | 10 ±
widget/kwhperiod/kwhperiod_render.js | 8 ±
If I create a new display and drop a new Kwhperiod value object on it, that kills the display.