How can I display round avatars in the forum again?
Searching in the ACP unfortunately didn't work.
I am 100% sure that they were round before.

There are two places you can set rounded images one for the forum and one for the arcade

 

For the forums you have to go to your themes and click on the pencil icon to the far right, find the custom tab and open it, the setting for rounded images is in there, you would have to do this for each theme

 

For the arcade go to Main Settings, View settings and the setting for rounded images is in there

Thanks for looking in the styles.