Here was a frustrating situation: After setting up a key for my dev box, I couldn’t connect. Putty and TortoiseSVN would die on an error of too many authentication failures. There was no additional information on the server other than this error, and, if I connected from another system, things worked fine.
The secret here: I had six active keys in pageant. I removed all but the key for my dev box, and now it works like a top.
