I don't know if this is already known or not, but it's possible to work around at least one version of the no-gui glitch. I figured this out after the glitch persisted after I re-installed KSP via Steam, started KSP with MechJeb2 as my only mod. I had MechJeb-related files in "Kerbal Space Program/Plugins". I deleted all of the contents of "Kerbal Space Program/Plugins" and ran Kerbal Space Program with MechJeb2 as my only mod, and found that the GUI worked. Then I added my large stack of mods back, and tried again, and it still worked. I just checked "Kerbal Space Program/Plugins" and it contains no MechJeb2-related files. I'm now using MechJeb dev build 270. (That's the version this work-around worked for). I used the 2.3.1 release when I re-installed KSP. I'm running KSP 32-bit on Windows 8.1 Pro(64-bit). I didn't check the KSP directory after my uninstall via Steam, so I'm not sure where the files in "Plugins" were from, why they were there, or why they aren't there now, but my MechJeb works now, and maybe this will help someone else.
