from looking at the pic, it seems tht your tension adjuster isn't parallel to you other wheels,( looks like its drooping from here)
that would easily make the belt run uphill towards the motor whilst running.
with regard to crowning, I think Arbalist was meaning the main drive wheels, not the adjuster. though while it may help the belt stay centralised, I think it would also ruin the flat surface I assume you require for a belt grinder.
as Arbalist also recommended, a tracking adjuster may well be what is required to actively hold the belt in the right place.
maby 2 rollers would be best, one each side of the belt, possibly fabricated into your tension adjuster?