Thanks for answering,
The recorder was set on 48khz sampling rate and 16 bitrate XLR stereo, the microphone was rent, but I think there was nothing wrong with the mic unless it really matters where you plug it. I did not know, if it mattered, where you plug the two microphones. Is there a rule, that says the center mic on the left channel of the recorder and the eight-shaped microphone to the right or something? I did not know, I just decided to plug them the same way everytime I used the MS.
My Tascam recorder gives out wav-files. I imported these wav-files directly into Logic Pro 7.2.3 and used the standard MS Decoder from the direction mixer plug-in, I did not change the base or the direction of the plug-in, for left and right seemed to be properly encoded, but a lot of the upper frequencies of the sound seem to be lost that way and also the stereo spread seems to be quite narrow compared to the original as far as one can tell - it's just as if the room suddenly becomes smaller. The recordings I made, were taken in a Tunnel.
Have a goog day. Christoph