I've looked into this but cannot replicate it with any of my 4 Yubikey's here. We don't do anything special and use the standard API. Here is an example of a key: ccccccgdnkiddllklhfiijelvtcgclbbkecbgkvithre
I'm sure yours are not any different. I just tried it in my account and it accepted the key without any issues. This is how it looks once accepted:
Is there a way to PM you a key entry or can you pull that from logs? It's probably ok to post here since it is a one-time password but I'll error on the side of caution. My Yubikey-generated-keys are 64-character strings consisting of all lower-case characters (no numbers or special characters.)
Here's a little more info. Using the Chrome Developer tools, I can see that there is a response to the key submission: "Invalid Key Format. Please try again."
Oops. I did not receive an email notification on your reply. Sorry about the delay.
It looks like your key has a different format. Not sure why, but all my keys works. Hence can you please send me some keys to my email address directly? Thank you.