yeah i took morph out also and it worked fine afterward. im going to do some testing and see if i can find out what the issue is with morph.
Edit: morph is also a mod in beta at the moment.
its not a problem with a texture pack i know that for sure.
i was thinking maybe someone left the game as a morph and now because of that you cant get back on? could be a possibility.
im also having the same issue and its pretty annoying, i read somewhere else it may be a problem with biomes plenty and only the mod author will be able to fix it.