I´m getting a headche just for this, i was trying to test some stuff for make a map. Everytime i use SDK shows an error, yesterday worked and i was able to make my custom model and it worked in TF2, but today the SDK dosn´t work (and with that the GUI studio does not work too) I already check the cache stuff and i reboot the computer and steam. This is the error :
I had this error earlier while tf2 was silently updating. VALVe did something with the way steam downloads games recently and the "Downloading/Updating" statuses no longer always display correctly in the library panel. Try updating TF2 manually and try again.
Did you repair the caches of both the sdk and TF2? I don't recall the appid, but 441 is probably the sdk and 4704 a game one. Or is it the reverse? I don't know.