Kerbal Space Program

Thanks for your report. InvalidOperation is indeed unwanted, and this particular error has been known to block editor functions in my experience. However, a game restart should clear this and allow trouble free operation. The exact cause and a good reproduction of the problem has yet to be found.

I can confirm that the craft can be opened, edited, and the fairing rebuilt multiple times without errors on my system which would indicate that it is something local to your system.

From your report, it appears that restarting the game does not fix it:
Is the same error thrown every time you attempt to load the craft and edit it?
Is this true even for a fresh start of the game, and creating a brand new save after copying the craft to the Ships\VAB folder?
Could you please run a file integrity check for KSP in the Steam client and let us know if anything is downloaded, and if so, if it fixes the problem?

Same error is thrown every time I encounter the problem, and I also encountered a stack overflow. I have been able to half-consistently reproduce it multiple times today (each time in a new session) with the problem save file, a new rocket that I was building, and also from scratch just using 2 parts, one object on top of fairing, even in a brand new sandbox save. I've been trying to find a pattern leading up to the bug, but sometimes I am able to rebuild the fairing twenty or thirty times without any problem. Then I will start over and it happens immediately. It seems to only affect the middle sized fairing.
Also earlier I said that nothing else was affected, but if a launch is attempted while the object is in that state, it hangs on the loading screen until client is forced to close.
Verified file integrity, 7 files were downloaded, no change.

Thanks for confirming the file integrity, and for making a fresh install. It certainly helps to rule out other variables.

I have managed to reproduce your initial problem once while randomly editing the fairing. We have a theory as to what maybe causing it, and why it is so random.

Possible Workaround¶

This may allow you to continue using the editor once it locks you out. However, please use with caution as it does not clear the initial error (read bad things may happen)

1. Open Debug menu (Alt+F12)
2. Select the "Cheats" tab
3. Select the "Show Input Lock Stack"
4. select "Clear Input Lock Stack"

Please report back if you have any other findings. As I have now reproduced the problem albeit as a random occurrence, this can be forwarded for further investigation.

No problem, I'm happy to help. I replicated the issue in a new save in a new session like you asked, here is the log and sfs files (I included a quicksave and persistent save, not sure if they are both relevant).