We're still struggeling with the error, but TAC provided us a workaround to export the needed data
mgmt_cli -r true show networks limit 500 offset 0 details-level "full" --format json >> FWMGNT_Export_Objects_1.json
mgmt_cli -r true show networks limit 500 offset 501 details-level "full" --format json >> FWMGNT_Export_Objects_2.json
mgmt_cli -r true show networks limit 500 offset 1001 details-level "full" --format json >> FWMGNT_Export_Objects_3.json
The export is limited to 500 objects. Therefore, we had to run it several times to get all objects. I've merged the files in PowerShell and exportet the needed fields.