Transmission 403 (Tomato 1.28)

I've been trying to get Transmission GUI working on Tomato 1.28.0000 MIPSR2-093 K26 USB AIO, but failed!

I keep getting a 403 Forbidden when I try to access the Trasnmission GUI page. I spent some time on the Web looking for solutions, but nothing solved the problem.

This is the content of relevent parts from my /tmp/settings.json

"peer-port": 51515,
"rpc-enabled": true,
"rpc-bind-address": "0.0.0.0",
"rpc-port": 9091,
"rpc-whitelist-enabled": false,
"rpc-username": "admin",
"rpc-password": "admin",
"download-dir": "",
"incomplete-dir-enabled": "true",
"incomplete-dir": "/.incomplete",
"blocklist-enabled": false,
"blocklist-url": "http://list.iblocklist.com/?list=bt_level1",
"rpc-authentication-required": true

Although I have set the "download-dir" value in the Tomato Web UI, it's empty in settings.json.

What do you think could fix the 403 and the empty "download-dir"?

Thank you.