I apologies- got it working!
I had two important obstacles, one related to the UBport:
1.Win10 somehow has a problem with talking to tablets and seems to require an extra driver, "MTK Driver"- I haven't verified this myself, but switched to Linux mint, where adb worked nicely and found the devices always. Anyway odd is that adb showed the device before I started Ubports, but later lost the connection somehow.

2.The TB-X306F could not get unlocked as described in the docu with "fastboot flashing unlock" but "Fastboot flashing unlock_critical". It seems, that command has changed somehow- my latest firmware before Ubports came was from 2024. No one talked about this parameter in the forums, but I found finally, that it seems to be an old syntax- may be Lenovo reused an old firmware, so that this came back. This changed command should be noted in the related documentation, not a problem of Ubport itself- it's just difficult to find, as there's no clear feedback that the unlocking failed partially, and Ubport itself only realizes it late in the process.

I have now Ubuntu 20.04 on it. Impressed, that I got it working, but it was not trivial.