So, I just replicated your configuration scenario including a new engine group, (including the umlaut), GIM domain, password repository. I did not see any issues.
HOWEVER, I did see the error when I changed the shared secret to something that did not match the entry as it exists on XMC.
On GIM, Administration -> RADIUS, make sure that the shared secret on that page matches the shared secret as defined on your GIM server as entered on the "Guest & IOT Managers" tab of the engine group. Communication, local or otherwise, will happen via RADIUS between GIM and the NAC engine. If the secrets between the two do not match (i..e, mine were both 123456, but I changed the secret on the GIM side to 1234567), you will get the following:
Error fetching Guest Users. Error on Access Control Engine/XMC.
Please check to make sure that your GIM configuration (Manager and GIM domain) on XMC is set correctly.