I'm having the EXACT same problem with the EXACT same setup---my reject rate stays between 20-30%....wayyyy too high---ive tried multiple pools with multiple difficulty settings. So far DGB256online had the lowest reject rate but I like thablockfactory more. tbh I like multipool the most but it had the highest reject rate.

i spoke with blokforge about it and they said that DGB pools are notorious for having problems with accepting difficulties and rejecting shares. they said it takes a day or 2 for the miners and pool to adjust to each other.

they also mentioned that they use this calculation in determining the difficulty to use. GHs/1.4----so running 3 741s on 1 controller would roughly be 22000GHs/1.4 which is roughly 15800.

About 3 hours ago i switched my difficulty setting to 15500 and so far the reject rate is about 17%---which is still WAY too high but much lower than it had been---im going to let them keep running for another 24-48 hours like this and see if things get better---im really wanting to mine DGB so im hoping this works---but only getting 70-80% of your hashrate just isnt sustainable---i will keep you posted over the next day or so on how its working---in the meantime it might be worth trying on your end as well.

also, if you or anyone else has found a solution please let me know! im dying to figure this out!