Thanks for your reply. I do run multiple monitors, but I still cannot find any hidden windows. Do you know what kind of window it should be, and what the process is called?
I once again did a clean install of TerraMaxx after removing the folders mentioned in the opening post, and the X-Plane log is now saying this: 0:00:39.957 G64: core: Run: [OnBoot]
0:00:39.957 G64: debug: GK: Auto license update is on.
0:00:39.957 G64: debug: GK: Update triggered. -18998.80 days remaining <= 2.00.
0:00:39.957 G64: debug: GK: Requesting license update..
0:00:39.957 G64: debug: GK: update_license()
0:00:39.957 G64: debug: GK: No valid auth token, manual login required.
0:00:39.957 G64: core: Run: [OnFirstFrame]
0:00:39.958 G64: debug: GK-IPC: No license: (maxxxp.terramaxx)
License challenge failed; TerraMaxx disabled.
When using Alt-Tab I cannot find any windows other than the usual ones. I will try to reinstall later (now I'm a bit fed up).