Good one! But more seriously, all of these boards/products have been in the same installation. Are you sure that is solid?

For example the GPS active antenna might be at fault? Or too close to other GPS antennas or other equipment (they do suffer interference in those cases)? Or a bad cable?

It seems highly unlikely that once they replaced the main board, there would still be the same fault in the board. You may need to debug your installation, at least until you can reasonably prove that it is not the problem.