Hello Tobias,
There were two shapes with the markers attached to their vertices which were causing the problem in the version 1.4. The lists of attached markers contain two markers that do not actually exist in the project and when the related shapes are selected for export, the saving operation fails. Problematic shapes, however, are skipped, if you use latest Metashape version for export.
I have sent you the script lines that will remove problematic shapes from the project and you would be able to export the shapes even in the old PhotoScan version.