Here was the problem:
When I installed Management Center, I don’t think it ever gave me an option of changing the path to the TFTP root directory, which defaults to (C:/tftpboot). I found where to change this in Management Center > Administation > Options > Inventory Manager > File Transfer, and did so, assigning the path for TFTP to another drive letter (b:/, on my server).
Of course, I had to manually create the tftpboot/ and its subdirectories on that new drive letter, which I did. As I already wrote in this incident, Management Center was aware of this new path, as I could observe it create the appropriate files in the tftpboot/configs/tmp/ directory to prepare for a switch to send its configs to the Management Center server.
The problem was that the file %install_dir%/Extreme Networks/NetSight/services/nstftpd.cfg remained pointed to the default tftpboot directory on the C:/ drive, even after I modified the path to the tftpboot directory using the Management Center GUI.
When I manually edited the nstftp.cfg file to reflect the actual path in which the tftpboot directory is located and restarted the tftp service on the Management Center server, I was able to successfully stamp configuration archives.
Thanks to everyone for your suggestions.