Unfortunately, MobYEhplorer doesn't really work on some SE phones...
I have tested it on z520i, z550i, k310i and v630...
of these, I was only able too delete the protected files on Z520, the otres zust wouldn't work using this method...

Did it eventually on the other phones, using far manager, must say that on k310i i had too debrand first....

Good luck on freeing u'r phone's memory...