It seems I was able to locate the problem, I found an answer on Flickr.

There is a mechanical problem with the camera. When taking of the objective and looking on the camera from the front, there is a ring with a small hook on it which is called the MC Coupler. When the objective is set on the camera, this hook 'tells' the setting of the aperture on the objective to the camera. This hook gets stuck on the right side, indicating small aperture. Therefore, the camera assumes there is low light and suggests a long exposure time. When mechanically setting the hook to the left and putting the objective back on, suggested exposure time changes as anticipated with the change of the aperture - until the hook gets stuck again.

To sum up: In the manual mode (M) of the X-700 the light meter should always give the suggested exposure time when changing the aperture setting on the objective. No pushing of the depth-of-field preview button is required for this. It is not working properly in this single case because there is a mechanical problem with the camera which makes the camera "believe" there is lesser light than there actually is and, therefore, suggests a very long exposure time. According to other sources, this mechanical problem is fixable, depending on what actually causes the problem. It might be that a spring has come loose which can be repaired by a repair service. It could also be caused by accumulated dirt under the ring which can be resolved by cleaning. It could also be that the ring has bent and either needs straightening or replacing.