I had the exact same issue with a k790i: Holding 2+5 works and gets me to the same place with XS++ and holding C did nothing.

I fixed it using SETool Lite.
Go here: REPAIRING EROM GUIDES for most sonyericsson phones - Top Sony Forums
Get the files from the "SETOOL 2 LITE 1.10 guide needed files" section and follow the instructions in the "SETOOL 2 LITE 1.10 erom repair guide dcu-60 or cruiser" section.

For me, at least, it was very easy.