Can't Change Author in topic

We upgraded our EE install last week to v2.10.1 - Build Date: 20150501. Sometimes we want to change the name of an author of a topic and before the upgrade this was no problem. Just select the right author in the publish field and hit submit. When i do that now it looks like the page is kind of refreshing, but doesn’t go to the view entry CP page. And the autor isn’t changed. I am a Super Admin and i tried this with several combinations and author names, but not changeable anymore.

Also did a check with the member group and author. The author is in the multi-author list and Super Admin must have the power to adjust this.

Any ideas?