This started just before the last major PS3 update what,2 weeks ago? When you go it either freezez you the min you click on a demo or a game or really almost anything. Either that or it boots you out of store and back to the XMB..
Alot of us are having the same problem,we have deleted store and reinstalled,,Have tryed Safe Start option 4 to rebuild data base,even try another they said to try by going to store using another acct. (my friend did that one and still no work) Even one of sony employees tole me he is having the exact same problem.
Right now im told only option it to go to store from your PC for time being.. Its the Store,,not our systems