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.
