I had messed up my mechjeb installation and this message at least informed me about it.

This error message should properly display the problematic path (it's hidden now behind OK button)

Needless to say, I've already fixed the installation issue

It displays fine here. It must do that when using some specific display settings. Did you change the UI scale or use some unusual resolution ?

Issue resolution tested in version 758 - messge displays correctly.
Do you have a plan to release official build anytime soon?

I'm using 1920x1200 resolution with UI set to default 100%. I've just reproduced this by copying MJ2 whole folder to a folder of another mod (exactly what I did before by accident). I chose a folder with a long name - "C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program\GameData\NavBallDockingAlignmentIndicatorCE".

My guess would be there's formatting issue with a long path name like this.