The signature check is very sensitive to content change, even white space or different line endings. All these intermediaries (the browser, copy/pasting, etc.) may alter the content.
I recommend using the the RPC commands instead to provide data feeds.
For example here is a batch script for windows that dumps your vote and signs it:
"C:\Program Files\Nu\daemon\nud.exe" getvote >"c:\MyDataFeed\vote.json"
"C:\Program Files\Nu\daemon\nud.exe" signmessage SedtEJa1dm6pSLagYXKvw5Sef3iH9VfwXC <"c:\MyDataFeed\vote.json" >"c:\MyDataFeed\vote.json.signature"
Replace the NSR address and the paths and save this to DumpVote.bat somewhere on your computer. Then you can just run it to dump your vote and the signature.
The Nu client must be running with RPC enabled (i.e. having server=1 and an rpcuser and rpcpassword in your nu.conf).
You can even add votenotify=c:\Path\To\DumpVote.bat to nu.conf to make the client dump your vote each time you modify it.
