Can't run multiplay in 1.8.8

My son upgraded to 1.8.8 and now can't use multiplayer. Single play works fine. As soon as you launch multiplayer you get a crash report.
java.lang.IllegalArgumentException: bad Base64 input character at 4019: 0 (decimal)

Launch multiplay

Minecraft crashes

Steps to Reproduce:
1. Try to launch multiplayer
2. Minecraft crashes
3. ...