Hi @Dobi
We experienced the same issue after performing an upgrade from Creo 7.0.x.0 to 9.0.x.0 a few years ago.
For some reason, this was caused by the fact that the 'older' Creo 7 workspace/server cache was also being used for Creo 9 and somehow this led to Creo always starting with an offline Windchill connection.
What fixed it for us, was renaming the root cache-folder and restarting & re-registering Windchill in the Server Management - thus this way forcing Creo to create a blank new cache folder.
In our case, the root cache-folder was the parent folder where the PTC_WF_CACHE folder was located (your location can be found in File > Manage Session > Server Management >> Cache tab)
Alternatively you could also set the following in the parametric.psf file of Creo 10:
SET=PTC_WF_CACHE=C:\PTC\PTC_WF_CACHE_Creo10
And optionally, set this in the parametric.psf of Creo 9:
SET=PTC_WF_CACHE=C:\PTC\PTC_WF_CACHE_Creo9
(If you start Creo using BAT-scripts, modify the lines that set the variables)