Hello,
I have the same problem with my Xperia T, the System UI unfortunately... popup comes up, and when I go to the control panel to manage memory (so I can erase the cache) the control panel crashes! Apparently Sony no longer supports this model, the flash module is not available for it. Guess that's why my internet provider included it for free in their subscription package...! :-$
I have tried reinstalling the latest version of Android supported by this phone, using PC Companion. Also tried restoring factory settings (after saving my files and data with PC companion) ; neither of these attempts did the least bit of good, the problem has remained in its entirety!
Tonite after work I will see if this page can help, maybe it could help your case as well :
https:// talk.sonymobile.com/t5/Android-development/Unfortunately-the-process-com-android-systemui-has-stopped-Sony/td-p/824449
Good Luck to both of us!
Daniel