So I did a quick search and found someone who had a similar issue that was never resolved.
Similar scenario, I've captured a few tapes using the AG-5710 over the last few days. Today when I went to do a capture, I get nothing but snow with audio, and the unit is indicating that it is an SVHS tape when it is not. I tried a tape that I successfully captured earlier in the week and same issue. Oddly enough, I had replaced another Panasonic deck (MD835 I think?) last year with the 5710 because it showed the same symptoms of snow with audio.
Does anyone have any ideas? The SVHS indicator coming on seems weird to me.