Omega B 2.5 DTI – 5.Gang jumps out when accelerating – possible causes?

Hi, with my Omega B 2.5 DTI with 5-speed manual transmission I had to make today at the mileage of 252,000 km on the highway the statement that the 5th gear jumps out suddenly when accelerating (shift lever jumps into the middle position). At the first time you still think that you have not inserted the gear correctly (unlikely, but at least possible), but if it happens then suddenly with every acceleration, surely there must be a technical problem. – 4 are completely normal when accelerating. Did any of you ever have this problem and can say something about the possible causes? I’m going to tap on a problem with the switchbar…