Could it be, that 2020.12 (Windows 64bit) is missing the meshlabserver.exe?
Or do I miss something?
I already reinstalled MeshLab but still no server to be found.
Meshlabserver is discontinued. From meshlab.net
MeshLab 2020.12 has been released. With this version, we dismiss meshlabserver in favour of PyMeshLab, our new Python library for mesh batch processing using MeshLab filters.
You can still execute your old .mlx filter scripts using a simple python script like this :
import pymeshlab as ml
ms = ml.MeshSet()
ms.load_new_mesh('input.ply')
ms.load_filter_script('my_script.mlx')
ms.apply_filter_script()
ms.save_current_mesh('output.ply')
If you really need the meshlabserver executable, try one older package.