The mixers obviously needs a repair, which probably means a replacement of the curcuit board dealing with input channels 12-20. You should open a ticket and ask Support for advice. They will give you a list of ASC's (Authorized Serice Center) which can do the repair and check the mixer generally.

I think those mixers had 3 preamp boards with 9(?) preamps each in them. The problem might be as simple as one (ribbon) connector working its way loose. Maybe worth the effort to remove the mixer's side panels for a look inside and to give all connectors a push.