If it's crashing the material browser, something is messed up with Hammer's files. Close Hammer/TF2, then validate TF2's game cache via Steam. Right click on Team Fortress 2 in the right side bar, then properties-> local files -> validate game cache.
Exporting maps is done via compiling, if it's not compiling then you have something that's causing the compile process to fail and that could be a novel's worth of errors.