Error 3203 restoring SQL Server backup: The data is invalid
Try running a RESTORE FILELISTONLY or RESTORE HEADERONLY. If these run successfully, that would prove that SQL Server is successfully accessing the .bak file. (And rule out some sort of permission issue.)

If that is successful, try running your RESTORE VERIFYONLY and RESTORE DATABASE with STATS=1. If it continues to fail in the same spot each time, it would probably indicate that your backup is corrupt, and you're out of luck.

If it fails in a different spot each time, it could be an issue with reliability of the location where your .bak is saved.