> AFAIK jack_client_new() is deprecated but exepected to work.

Yes, it's not an "error", just a message/warning. Hence, "generic" (and not helpful).

Anyway, myth busted. Problem is not entirely wine. Partly machine, partly wine, partly software, maybe. Why? AmpliTude Live (standalone) works. If Guitar Rig does not, it's gotta be the intersection between machine, wine, jack and itself :)