I've had this problem for a few days now and been trying to figure out the cause myself, but so far no luck as I am trying to run a heavily modded version at the moment. The problem is that when I enter the VAB or SPH and create something... anything... I cannot exit the building after. Pressing the exit button does nothing and I have to force close the game. Everything was running fine until a few days ago when I noticed BARIS was updated and I ditched Kerbalism to pick that up, as well as USI Life support and USI Kolonization as well to fill the life support gap left by the removal of Kerbalism (I also added MK33 as well because I like how it looks). After I did that I had a problem where Kopernicus wasn't recognizing the solar system on game load, so I had to uninstall the game and reinstall it and all the mods again, and that fixed that. However, upon starting a new save and going into the VAB I found I could not exit it again. Thus this odyssey began.
I removed the new mods I had added and restarted a new save, but found this problem persisted. I uninstalled KSP, did a clean install, added all the mods again and still the problem. I've done this a few times now. Last night I made a clean install and added everything except for Kerbal Construction Time, and the problem was gone. Adding it back in and the problem came back. This morning, on another new install, I added the mods I thought were the problems one by one and tested after each install. First KCT, no problem. Then Baris with KCT, and no problem. And I did this for a bit longer with more and more mods. However, I am running a loooooot of mods and it would take too much time and I am an impatient woman, so when I figured the problem wasn't from the mods I suspected I installed the rest of my list and loaded the game to find the problem back. Which brings me here, to you lot who know a whole lot more about this stuff than I could ever hope.
I have tried to check my logs (which, btw are harder to find than the sticky on asking for support on this forum suggest) and tried a few different things. Was getting nullreferenceexcptions from mods like DeepFreeze (recently updated) and HangardExtender, and removed them but it did not fix my problem. I really hope someone here can help me out, because I have so many plans and dreams of what I want to do in KSP right now but no way to actually play the game game.
I'll include my logs, but as I stated it was hard to find them and I'm not sure they are the correct ones, so if they aren't any help locating them would be nice (I don't actually seem to have an output log no matter where I look for it).
Edit: I just removed BARIS and went back in to test that. At first it seemed to be fixed, but I actually forgot to hit the Launch button and so KCT didn't take over and begin the building process of the ship I designed. So after doing that I tried to exit and found I no longer could. Which is weird, until just recently KCT was working perfectly fine, so not sure if there is some kind of conflict I introduced or not.