Share your experience!
Boot on safe mode and check again
http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008
Also check
settings > apps > all > scroll to the bottom, perhaps they are disable, do you have any antivirus or taskiller running?
if nothing work, repair your phone with PCC
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
Keep us updated
I'm in the same situation right now.
I tried booting in safe mode, message still doesn't respond.
I'm backing it up now just repair the phone.
Will keep you posted.
PS.
Aside from the messaging problem, the phone also comsumes so much battery power. Like 60% to 7% in just 3 hours.
-Christian
UPDATE:
After I repaired the phone through PCC, the message app worked. So I recovered the backed-up files (through PCC again), and the problem with the messaged app occured again. I did a safe mode, the problem still persist. The problem probably lies on native sony apps on my backed up files.
If you have a lot of sms/mms (1000+) in your message database it can take some minutes for the phone to index the database and behave normally after an upgrade.
Have you tried simply letting the phone stay booted without touching it for maybe 20-30 minutes after update?
I know it is a longshot but if you haven't tried it, it is effortless to test