crashes and sends me back to launcher. 1.7.9 / 1.7.10

it wont log me into our server and crashes me out back to the launcher

What I expected to happen was...:
it to log in like usual

*What actually happened was...:*the server is in 1.7.9 and usually works, but when i try to log in it kicks me back to the launcher with a report

Steps to Reproduce:
1. edit profile
2. ...set to 1.7.10 or 1.7.9
3. ...go to multiplayer and try to join our private server
4...... it kicks me back to the launcher after trying to load