Wow, thank you for that detailed explanation. I'm afraid the printer has been bad engineering from the beginning and I didn't make it better with my tinkering through the years. But unfortunately I currently can't afford a good new one. So it seems I have to redesign the carriage (once more) to make the belt parallel to the axis.
As a test I removed the motor and installed only a pulley with a bearing inside to see how the belt afflicts the axis without the actual motor attached. I don't have a spring balance so I can't actually say how many Newton of fore are required to move the axis but the resistance was very low and therefore can't be the root for my step loss problem. Also the belt has in no position been loose enough to threaten to skip teeth of the pulley, so I guess I can rule that out as well.
As a test I removed the motor and installed only a pulley with a bearing inside to see how the belt afflicts the axis without the actual motor attached. I don't have a spring balance so I can't actually say how many Newton of fore are required to move the axis but the resistance was very low and therefore can't be the root for my step loss problem. Also the belt has in no position been loose enough to threaten to skip teeth of the pulley, so I guess I can rule that out as well.