Hi Michael, Thanks for your reply.
Using "" gives a string error, you can not set it like that.
Using "none" also doesn't seem to work
Using @{remove=} (like you have to do in Set-Mailbox to remove -grantsendonbehalfto entries) also doesn't work.
What I have tried now is just removing all users from the old group (thankfully I used a group specifically created for the testing) but we keep getting 4662 events from the old config. Maybe the server needs time to replicate, I will give it some time :).
Still find it strange that I'm the only one with this issue. Thanks!