Its a known bug now.
Workaround: downgraded to v 11.20.2 where it works
or got this from DeveloperSupport:
1. Run the query below:
select * from tblNotificationSubscription where SubscriptionKey ='feature://notification/' order by UserName
2. You may find there was one "duplicate" where one email started with a whitespace and one not.
3. Please set Active = 0 for one of the duplicate records, the issue will be gone.
Issue is still present in 11.20.4.. Is it one that is being worked on? If it is being caused by content then how do you fix it through the CMS?
CMS bug is CMS-18303
But i cant see it is official yet.
If in azure you can send it to dxc support, so they can do the db hack it before the release, then it works.
You can also remove all posts in tblNotificationSubscription which starts with whitespace in email, thats also an options.
The script needed to update any identified active users with leading or trailing whitespace can be
SET Active = 0
WHERE pkID='[name of identified id's]';
Just upgraded to latest CMS 11.20.3 from 2 feb.
seems an error in
I will try to downgrade ...