I have had this happen when some field is not set or is set incorrectly. Either on the settings tab or the data interfaces tab.
When you do the pulldown for the Default Server Group, is your server under the Related Server Groups by Protected IPs section?
If you attempt to change it, do you get the warning popup about removing assigned services? And if you click OK, then you get a message about assigning services on the data interface tab.
I was able to change an agent to not have a server group, and then back to the correct server group without issue.