Hi there, 99 FD here, JDM model so it comes with black box, stock twin turbos on power fc.

I'm having very strange issue. There is a vacuum line, on the diagram it is called "vacuum supply to the black box", I highlighted it with blue here:


The issue is, if I leave this line UNPLUGGED, so it just hangs in the air, everything works as expected. I literally wasn't aware there are any issues or popped vacuum lines because both turbos boosted fine and I experienced no issues.
However, if I correctly plug it to the box, suddenly issue appears with first turbo. Car works fine, secondary turbo boost fine (0.9 bars), but primary turbo doesn't really wanna boost (only does up to 0.1-0.2 bars).

Any idea what may be the case? For the moment I'm going to leave it unplugged and have working car, but I'm pretty sure this is not really the long term solution and something else might be wrong with my vacuum system? Any suggestions how to diagnose this?

This is how the line looks from behind the UIM:

 

Dunno! If the hoses are hooked up correctly, I'd be checking the one ways in the black box. They also do have a rep of developing leaks with age and causing mysterious issues, a soapy water test might be worthwhile.