That's disappointing to hear for a product like this.

How did this happen after the most recent update?
And why is this not failing for the other domain (they use the same user database)?

I mean they literally use the same table for wp_usermeta and wp_users in the same MySQL database. So why would that fail for one, and not for the other?

Fixed it myself ... thanks for the hint where to look.

This is how I fixed the issue - wasn't that hard once you know where to look.

1. Made a backup of the tables wp_usermeta and wp_users.

2. Determine what records in wp_usermeta is no longer linked to an existing user (NULL).

SELECT *
FROM wp_usermeta LEFT OUTER JOIN wp_users on wp_usermeta.user_id=wp_users.ID
WHERE wp_users.ID IS NULL

3. If all looks good, remove these rows 

DELETE wp_usermeta
FROM wp_usermeta LEFT OUTER JOIN wp_users on wp_usermeta.user_id=wp_users.ID  
WHERE wp_users.ID IS NULL

4. Test and verify if everything works as it should.