This is easily fixed. Go to your database thru phpMyAdmin and delete the matching entry from the anti-spam table.
I had the same problem occur with an entry recently and this fixed it.

Tried to update the antispam and I'm getting this strange error ... anyone know what to do?

Remote error: Invalid return payload: enabling debugging to examine incoming payload (2)

Remote error

That means it's a temporary problem at the remote server. Try again in a few hours.

Graham wrote:

Remote error

That means it's a temporary problem at the remote server. Try again in a few hours.

However, it's been like that for more than three weeks. The last valid update I was able to update was July 10 ... It's now Aug 4

Gotta be a problem somewhere on your end. Your host perhaps? Having the duplicate entry means something went very wrong, so perhaps your corrective action(s) got things impossibly twisted? You might consider three steps towards resolution (assuming you have phpmyadmin available through your webhost):

1: export your antispam table to a file.

2: open the exported file (wordpad will work) and copy the part that says "drop if blah blah" and also rebuilds the table.

2b: NOT the "insert blah blah" stuff - just drop the table then rebuild the table structure.

3: in your evo_settings table edit the 'antispam_last_update' to be '2000-01-01 00:00:00'

Now do the antispam update thing again and keep your fingers crossed. (Alternatively you can participate in whatever method(s) your preferred belief system has enabled to increase the likelyhood of a positive outcome, but since I go with luck I use crossed fingers.)

If you still can't get an update it's possible your host has done something funky that is not allowing the update to happen. I don't know what or why, but it's possible. Good thing you got that exported file eh? Go back to your phpmyadmin, copy all those 'INSERT blah blah' lines, and use SQL to rebuild your table the way it was. You *should* be back where you started, meaning you won't be able to get updates but will be able to add your own spammers to your own antispam table.