In this situation, the target accesslog file is occupied by ProxySG and you need to rotate the file before downloading it. Please try following CLI command for the specific accesslog profile.
Enter configuration commands, one per line. End with CTRL-Z.
ProxySG#(config access-log)edit log main
Please change accesslog profile name (in this example, editing "main" accesslog profile) which you would like to rotate
ProxySG#(config log main)commands cancel-upload
ProxySG#(config log main)commands close-connection
ProxySG#(config log main)commands rotate-remote-log
If above commands don't help, restart ProxySG to force rotating the file. Then you can re-visit the above URL to download the accesslog.