In sandbox I have enabled opportunity teams ans opportunity splits. This is necessary to use custom currency fields in forecasts. We don't use opp teams and opp splits, so I have made a "split" that's 100%. Now when I test this and make changes to the stage or the amount of an opportunity, I get the error: "You do not have sufficient access to modify splits of this split type." even when logged in as admin. I tried to modify access on opportunity splits, but this cannot be changed. 
Did somebody elke encouter this same behaviour? We would like to use forecasts, but this error is keeping me from enabling the dependent functionality in salesforce.