I started to get this error after installing the new firmware of D3 so I did exactly the same Mike showed me how to.But Didn't work! then I realized in the www/pages/cgi-bin there is 3 file connecting each other and this three are calling each other.

"upgrade.cgi"
"upgrade_clear.cgi"
"upload_conf.cgi"

so I edited with this files in ssh editor I searched in existing "Cannot Find Signature" so I put # type before words as like Mike's codes. and worked for me. Thank you again Mike. Yes I think with the latest firmware having strong security with this 3 files I think.