I get resolution error message on my Onkyo 1008 whenever I set the media player (PCH) to a PAL setting that is not supported by my TV - which implies that the Onkyo is correctly passing through the signal that is sent out from the source, but my TV can't display it.
My suggestions:
1. Try to upgrade the FW of your Onkyo to the latest one. I did this the 1st time I powered my 1008, and there was in fact an update available (as of May 2011)
2. Have you tried setting all sources and the Onkyo to 1080p? I noted that your Plasma is 720p but it should be compatible with 1080p input via HDMI, unless it's a very very very old model.
3. All your sources (NMT, DVD, etc.) are also set to 720p right now? I think I've seen transient resolution error when I switch from a 720p source to a 1080p source. Transient because the error is gone once you hear that characteristic TIK TIK sound of the Onkyo as it recognizes a signal from a source.