I think this is a problem of the stratum of yiimp.
I have a yiimp pool (pool.ivema.es) and get the problem that when I put the priority of the stratum less that 1, I get a lot of rejected shares, most of them invalid and low difficulty shares.

I investigated and could not find anything, but is something related to yiimp stratum for sure. I tried a mpos/nomp pool and it worked fine with 0.1 diff.
But i like yiimp because you can add more than 1 coin...

Perhaps owner of the official pool can gave us how did they get to use 0.1 diff on the yiimp stratum....

Thanks,
Witxo