I have also remade the whole thing. Put some magnets into the jig to make the saw cuts even more exact. Tried different pieces of wood. When using the jig for 90° degree cuts everything works perfect and i get dead square right angles.

The suggested general approach for handtools is not to expect perfection with the first step. For the specific need of a 45 degree angle you'd want a shooting board for a hand plane in a similar vein to your current jig.

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Ideal for fastener diameters of 1/4 and smaller, long engagement length fasteners, and components requiring occasional adjustment. Apply to threaded parts before assembly. Controlled lubricity for accurate clamp loads. Parts can be disassembled with hand tools. Approved for use in or around food processing areas.

An alternative to cutting accurate 45 degree angles when you want to have the finished joint at a 90 degrees is to make sure that the cut pieces result in a 90 degree angle but not worry about each of them individually being 45 degrees. So if you jig consistently produces a 43 degree angle, cut one piece that way and the other piece rotated 90 degree to have a complementing 47 degree angle.

All the angles measured are perfect. I cut very slowly to avoid to much play from saw. The piece of wood is fixated with clamps and doesn't move during the cut