I changed to the new config fomat for the disabled option in ~/etc/check_mk/conf.d/wato/contacts.mk and removed the comments in /omd/sites/rh/lib/python3/cmk/base/notify.py.
As a result, I dont get any error messages.
But when triggering a custom notification, the problem of multiple notification emails persists.
Did the werk #6417 fix this issue of multiple messages for you @mho-nicat too?