[Plugin: User Avatar] Error when using with WP Multisite / Or error on uploading image

I activated the plugin on our wp-multisite installation on site-level for all multisite-blogs.
Then I went to one multisite-blog and tried to upload a picture.
The upload-iframe shows this error: “Du hast deinen Speicherplatz verbraucht. Bitte lösche Dateien vor dem Hochladen. Zurück” – translated: You dont have enough free space. Please delete data before uploading.”

The space is not the problem. I suspect a problem with the file-upload-directory for multisites…

Anyone knows a fix?

Thanks

http://ww.wp.xz.cn/extend/plugins/user-avatar/