I gained nearly as much on the sign-up bonus playing blackjack and cashed out without issue. I've also bet well over $100 per hand while playing the monthly bonus without issue. This is the first I've heard of such a problem. Two possible reasons for the difference are: attempting a cash out before completing Neteller wagering and making a large gain while using large bets.
If you do not have luck with 888 support and Casinomeister, try eCOGRA dispute mediation (888 is on the eCOGRA approved list) --
http://www.ecogra.com/dispute.aspx?Page=3&OP=P