I had trouble dumping the priv keys with the new wallet and many times had to request it twice, sometimes it worked first time and others not, not sure why that was, the wallet was unlocked, maybe it's just me.
Otoh I remember had the same problem recently with one privkey...wallet was unlocked and was getting error -4 if I remember good.... but don`t remember how I resolved it.... aaaaaaaaaaaa
i do!
I wanted to import privkey on the wallet where this privkey was already imported.... check listaddressgroupings if the wallet address you want to import is already there?

Think it will help you

Cheers.
btw. masternodes updated thnx Flare
Thanks, yep I've had that too in the past, but this time it wasn't re importing priv keys, but dumping them, anyway it worked the second time that I tried each, no idea why it sometimes took two tries to get them though.