Cause
If the file can be transferred from a different PC, the PC itself may have a problem.
Solution
If the file can be transferred from a different PC
The PC that you tried to transfer the project file from first, may have a problem. Uninstall the USB cable device driver, and then reinstall it. Also uninstall GP-Pro EX once and then reinstall it.
If the file cannot be transferred from a different PC
The USB cable may have a problem. Transfer the project file by other transfer methods, such as by using a LAN cable or external storage.
34.3 Transferring Project Files via Ethernet (LAN)
34.8 Transfer using external storage