My active sync account just deletes itself every few days (twice yesterday and once this morning, this is the worst it has been). Everything works great and then one day, POOF! the account, contacts, calendar events all gone. I then have to recreate the account... very annoying!

 

I should add that I also have the Galaxy S2 running 2.3.5. Mine is the Skyrocket version.

 

Go to settings, applications, manage applications and clear data and cache for the app. You will have to add your details again after but hopefully it will sort the problem

 

Although the solution should be the same for both devices I will add that we have a forum dedicated to the skyrocket here
Samsung Galaxy S2 Skyrocket - Android Forums

You should find lots of helpful info specific to your phone there.