Plate solving using second scope fails with local solver

I use Plate solving with an SBIG through a 12" RC (f=1600mm): works perfect.
When I want to use my Canon EOS on a 8"Newton at f=560mm plate solve fails.
Is there a need to give any scale (px/arcsec), or not ?
I cant find any setting.

Thanks Ralf

It depends on what plate solver you are using.

Hi Ken,
updated to SGP and use ansvr 0.15.
ansvr 0.15 config: scale error estimate was blank (as told somewhere in the forum). Didnt work, tried 5, didnt work, tried 100, works well quite now, but dont know why. I that right? Does it now work also with the second scope, too?