This one helped me to solve current probelms: https://confluence.atlassian.com/jirakb/user-is-not-valid-for-this-user-picker-1047554037.html
But question is, why this only appeared in some projects. The field it self is NOT used in any screen but in 2 out of about 40 projects the error appeared.
To avoid future errors I would like to understand the background.
Hello Community! We thoroughly enjoyed this just-for-fun conversation in the Jira Admin Group about what it's like to be a Jira Admin. For #JiraJuly, our talented designers created these graphics t...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events