Just to keep you informed: this is a (now) known defect.
I got rid of this message by changing br/xml/cli/Group-Additional.xml (of course after making a backup-copy of this file )
Just deleted the complete complex_command-item with the command_id="addPermissionToGroup-1" (the one with four "java.util.String"-parameters).
This is up to now *NOT* an official workaround - but support is testing if it could be one.
It's still not official but here are the last news from my side. Support told me:
"Internal investigation shows that the change you made will work as long as there is no other command which requires that method. A definite answer can only be given once Engineering looks through the defect and analyses all dependent parts."
There is a defect opened with the following number: QM001692664
So I will set this to answered.
Thanks for the info... this one has had us stumped for a couple weeks now, too.