20-25% (up to 50% on 5 min average) of the submitted shares. This occurs with both the Nicehash CPU miner as well
as cpuminer-opt with the Nicehash mod. It also occurs on all miners at the same time.
It seems to be clearly a pool issue. Is it being investigated? It may not hurt buyers very much but it's a significant
penalty for miners.
Where are you from? Note, that we do not have HK and TK proxies up for cryptonight yet, you are being directed directly to US server.
We also checked some miners (top ones with highest speed) and there are some miners with high stale issues. Since there are only some and there is no correlation between them and because some miners are completely clean (especially big ones), we assume this must be a networking issue between us and you. Also, a stale share happens when miner is "too late", which also suggests a networking issue.
What you may try is to mine on different server, EU/US.