Did I get it right that when you create a new collection (not a smart collection which is not supported) the check-box to enable the collection for sync is not available. - Guido
The top left shows my name and also that sync is 'on' but no check box or if I create a new collection there is no box to sync collection.
I can quit and restart LR and the option come back provided my sync is on.
I have no idea when it goes away
Have you tried to re-sign-in via the Creative Cloud app -preferences. So quit Lr Desktop, re-sign-in via the CC app and start Lr Desktop afterwards. Would be interesting to know if you see any difference.
Yes tried that and resigning in does work, also just restoring LR is OK providing sync is on. Mystery is why it just randomly stops even though sync show on.