same problem ({"code":-32501,"message":"Share rejected due to low difficulty"}) for me too.
bminer 12.1.0 at grin-pool.org worked perfectly since 2-3 days.
is this low diff problem not also a huge think at grinmint.com pool since yesterday?
yes!
Working on it.. the problem is that I always have problems to connect to grinmint / grin-pool, which makes it hard for me to validate.
Will be fixed soon.