I was suspecting a mpos problem, but after investigation it seems a daemon problem.
For unknow reason the daemon gives a total balance that is less that the one you see if you open the qt wallet.
Maybe this situation let mpos give strange behavior has it sometimes go to negative wallet balance that is a nonsense.
I put all donation coins to the pool, so balance goes positive again for mpos and it now run almost good.
Actually the balance is 880 GRW for daemon (and only 509 are reserver for user payout) and 2780 true value in qt-wallet!!
Maybe a delete of all the growthcoin database in the server and let it rebuilts from scatch will resolve the balance problem in mpos, but this means hours of stop.
As I don't know why the daemon has such problem during the two day ago night (that maybe has produced orphan blocks), I upgrade the server today and now it it more powerful