Samsung Galaxy S II Skyrocket

I purchased this phone on January 2nd, and it worked fine. About 1 1/2 weeks ago it began showing the "DM initialization is not ready. Check network or SIM" whenever I tried to check for a software update. I took it to the AT&T store today and they replaced the SIM card with a 4G LTE SIM card; however, when I tried to check for a software update a few minutes ago, it gave me the same message so I had to restart the phone in order to complete the process. Does anyone have a clue what could be causing this error? The phone is not rooted.