Publish Services to Adobe Stock always producing error message

I've often had a problem when trying to Publish to Adobe Stock getting the message: "Upload failed. For the Adobe Stock plug-in to be active, you must be signed in to Creative Cloud" despite most definitely being signed into the the CC.

I could usually fix the problem by signing out and then signing back in again. However from yesterday this is not working and I cannot now Publish to Adobe Stock. I always get the same error message.

I've tried reverting to CC7.2 from 7.3 and the result is the same.

I've tried switching LR CC sync off and on. I've re-loaded LR CC, re-booted my iMac - with no success.

Any ideas would be very welcome!

Thanks