Problem after window replacement | Measured incorrectly?

Except for the VWS, there is no structural change to the house (at least none affecting the windows). The window measurer had all the plans on hand, which showed the correct clear dimensions of the wall reveal (without exterior plaster). This plan was apparently ignored and only the measurement including plaster was taken.

The window fitter had all the plans available that listed the correct clear dimensions of the wall reveal (without exterior plaster). Apparently, this plan was ignored and only the measurements including plaster were taken.

If that was part of the contract, you have a chance. Have you already accepted the service or complained in writing with reasons?

The windows have not yet been paid for, nor has it ever been confirmed that they fit. Since they were installed, there has been a dispute about who is to blame.

that an 18cm VWS is to be applied to the exterior facade

Proven or oral?

The window measurer had all the plans available, on which the correct clear dimensions of the masonry reveal (without exterior plaster) were indicated. This plan was apparently ignored and only the measurement including plaster was taken

Could it be seen from the plan that thermal insulation with the corresponding dimensions would be applied?

Did you report the defects in writing with proof or only orally?

No, everything was handled by a window installation company,

You mean: a janitorial service that does not also trim hedges?

This would only work if all costs are covered by the window company?

But please only the costs. However, legally that will be a tough path, because even a bungler is allowed to do rework first.

Why are there trained professionals then?

If competent people among the window installers were not even rarer than among the real estate agents, I probably would not have had to turn my back on the window industry.

The window measurer had all the plans, which showed the correct clear dimensions of the wall reveal (without exterior plaster). This plan was apparently ignored and only the measurements including plaster were taken.

I suspect you mean structural plans with target dimensions. But firstly, these are not the same as a structural measurement (which I have yet to see in the wild, it probably only exists in Wolperting), and secondly, here the as-built measurement after plaster makes more sense – only the info about installation in an existing building should have been given to the professionals in production(planning).

It doesn’t come out of nowhere that I suspect a real professional (pun intended) company ;-)

Since these were installed, there has been a dispute about who is to blame

I wish you all the best. The search for the culprit is almost always a dead end and prevents finding a solution. Quickly, the parties care about saving face and winning besides the matter itself. Then the situation is stuck. If you want a good solution, you have to give the other party a chance to agree to it without personal losses (not money). Clarifying a blame question is over with that. Twice unlucky.