I found this issue is caused by RTmedia Plugin (former Buddypress media). Once disabled group avatars uploading went fine, once activated again group avatar uploads did not work anymore. I will post this issue on their forum
I have the feeling it has something to do with ownership of the files. When I look with Filemanager within DirectAdmin to the files in de uploads directory , for example: uploads/group-avatars/8/<ipload filesnames> I see that the folder 8 and the files names are ownership: Apache, instead of my domain username…. Any clues how to resolve this, if it has anything to do with it?
When I look in the normal avatar folders al filenames and folders are owned by my domain user.