Ok, went to the file domain_mapping.php and make a output of “$current_site” just before the the error is thrown. It seems like $current_site->path is NOT set to “/” in my case, instead it’s set to nothing. A quick and dirty solution is to edit the plugin-file like this:
change
if ( $current_site->path != "/" )
to
if ( $current_site->path != "/" AND $current_site->path != '' )
[EDIT]
Better solution… Seems like the there is a problem in the wp_site. Go to the database, find the wp_site-table and set the “path”-column to /