I have already tried without success to:
- Re-add the widgets (it has worked with two of the loading widgets, but not with the others)
- Clear the cache
- Re-Reboot
- Uninstall and reinstall the problematic apps
August 20 update: the problem seems to have solved itself by just waiting?
Since I last commented on this post I haven't rebooted my phone or attempted to re-add my widgets. Today I tried again, just to see if something has changed, and they actually loaded after a few seconds. I suppose it's fine like that.