Failed to load mods: UndergroundTransport/data-final-fixes.lua:28: attempt to index field 'sheet' (a nil value)
stack traceback:
UndergroundTransport/data-final-fixes.lua:28: in function 'makePort'
UndergroundTransport/data-final-fixes.lua:99: in main chunk

Mods to be disabled:
• UndergroundTransport (0.1.8)

This looks like a mod compatibility issue - what other mods are you using?

Ah. okay. I am currently at 330 and I doubt you wanna see that list haha.

Looks like https://mods.factorio.com/mod/EntangledBelts is a potential culprit here, at least in my case.

EDIT: Nvm, re-reading the error, it's a different one.
Failed to load mods: File UndergroundTransport/graphics/output-eb-express-underground-belt.png not found

Ah I see the problem here - Underground Transport tries to build underground equivalents for each belt it finds in the game but only has sprites for the default set, so mods that add other types causes this problem. This is great for mods that add faster belt variants but not ones that alter the default ones. I think the fix is to ignore any belts it can't find sprites for, assuming that's possible.

I've fixed this in v0.1.10