it is a IDC connector i de-soldered and clean/checked underneath and reinstalled it. at the end beeped all the pins from board to board..
(was 100% ok but now for sure.)
as i told above i ordered a HV518PJ because it is positioned at a terrible place to probe and cost nothing to replace.
But i would more suspect one of the demuxers or the multiplexer chip. but first wait for the VFD driver to arrive, maybe it is an easy one

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and my new frontcover is also on the way.