Sorry for your problems. Go into settings, from settings go to applications, from applications go to manage applications. Select ALL at the top of the manage applications page and look for Youtube, its in alphabetical order so it should be at or near the bottom. Select clear data, and then force stop the application. Take your battery out of your phone and then put it back in. It should work after that. If it does not, call us at 1888-211-4727 and we will be able to assist further.
This is a known issue with Evo, one way of watching videos in youtube is by selecting Internet as your player program when asked, and not youtube app. If you go on older pages of this forum, you will see more info regarding this problem. http://community.sprint.com/baw/message/140114#140114
I hope this Helps .