I tried manually overriding to get this working in 2.1 experimental and there is an error when upgrading underground belts related to this code in control.lua

if entity.type == "underground-belt" and entity.neighbours then

Seems this was renamed in 2.1, it's now entity.underground_belt_neighbour.

Thanks for the hint.

Unfortunately, I'm currently not doing anything related to factorio in the next weeks.

No worries, I got it working by making the change myself to my local copy, just wanted to save you the debugging time as and when you get around to updating the mod. Other than that it seems to be compatible with 2.1 as is.

Actually I see it's on github, I'll submit a PR.