how can I release players or extend contracts, when i´m playing in sandbox mode?
( I already deactivated cpu control)

Should be able to click Edit Player to edit their contract. And click the X in roster management to release them.

I'm digging up the topic because I have the same problem of not being able to extend contracts in Sandbox mode. The button is always greyed out, even on players with 1 year left in their contracts.

I know that I can edit the contracts, but that eliminates the whole negotiation process and I have to think of a contract myself which I can theoretically can put any amount of money in.
Can this be fixed in a future version?

I thought if you clicked on a team in the top bar it "logged" you in as that team so that you could operate as normal. Could be there is a scenario that doesn't work that way or you're clicking on the player outside of the team screens... in which case you could try clicking on a player by navigating to the team some other way.

No, I tried to navigate to the Team in different ways, through Standings, the navigation bar, next Opponent etc., and to Players in the Leaders and the Search Screens, but it just can't be done. If I move from one Player Card to another in the roster screen, I even see the Contract Extension button pop out and get orange for a split second if the next Player has one year left on his contract, but it stays greyed out after that.