think the ecu is protecting the engine because of an error?

can also be a faulty sensor. Don't think it is programmed at 2500rpm  max as a standard rev limit.  Opel does have settings for rpm, but more in the likes of idle adjustment.

 As said by other member the ECU has put the vehicle in limp mode due to some faults in Engine Management System.........So, the best way is to Diagnose the Engine System for any fault codes...............Most common faults on this model are EGR System..........if you re-cycle the ignition do you get the power back to normal for few min and go back to limp mode again.............?