All, got an issue with the above, my external users can suddenly no longer add their names into a sharepoint online list.
The people picker resolves and finds them. But then you get “this field can not be left blank”
Had this issue last year on SP 2016 on-prem instalation. After 1 hour discussion with the system admistrator - some magic with User Profile Synchronization serivce was done and the the issue was fixed. Actually i remember that i traced the HTTP request which were related to the loading of the people picker and i found that some of the fields (do not remember which one) were empty.