problems with invalid shares could be due to very different number of currently active miners on YIIMP. Can you give us estimate number of miners for XVG on YIMP and on ZPOOL at that time you got errors?
The YIIMP x17 stratum is always full. Both ports. 3500+ miners typically. If you can connect on the first try you're lucky.
Invalid shares doesn't ever stop happening. It is always an issue and at about the same frequency. I don't have hard numbers however I get seemingly at least four or five over each hour of mining with two 1080ti/x17. Payouts are still OK - so it's not a huge issue. I just don't like seeing the red "booooooo" come up on the client lol
Zpool never shows invalid jobs like this for me however it has other problems. I've switched since to phi algo and am on altminer.net now.