Quote from Another Bad Pun on November 27, 2011, 10:14 pm

Alright, I've figured it out. As it turns out, Portal is not compatible with Source MP; only TF2, CSS, and DOD can use the MP version. Switching to Source 2009 fixed the problem. I have to admit, I feel pretty silly now, because this was stated in the Source MP update:

Quote:

- This update addresses all know issues introduced over the past week including:
* Crash when opening the VGUI model browser in Hammer
* Hammer lockups
* Black textures in Hammer, HLMV, and Faceposer
- Added a new Engine Version called 'Source Engine MP' which is the proper one to select if working with Team Fortress 2, Counter-Strike:Source, or Day of Defeat: Source. Users should select 'Reset Game Configurations' in the SDK Launcher so that these games become properly associated with the new engine version.
- Tools binaries for Portal, Half-Life 2 and its episodes are now located in a directory called 'sourcesdk/bin/source2009/bin'. Users may need to update shortcuts they've created for vtex.exe, studiomdl.exe, etc. in order to ensure they are using the proper versions of these tools. The binaries used for modifying Team Fortress 2, Counter-Strike:Source, or Day of Defeat: Source remain in the 'sourcesdk/bin/orangebox/bin'.