open *username*_settings.DayZProfile in notepad.

Thank you very much Dzizuzs. Yours was the correct answer and this annoying issue is fixed now.

Everytime I start DayZ, I see this glitched UI.
I can still play, though. Just choose a server in the glitched UI and join.
When I then exit the server, I see the normal DayZ UI and can choose characters and play normally from there.
I've taken all my characters to different servers and moved a bit then exited.
Still I get the glitched UI everytime I start DayZ.