hello,
thank you for your response. I purchased the yubikeys from
http://www.collectivesoftware.com/ They are (three) white yubikeys with firmware 2.4.1. I programmed them with Yubikey Personalization tool, version 3.1.11, library version 1.14.1. I programmed them on 02/14/2014 (at 10:43am) one after another through "advanced" OTP function. I registered them with the yubikey server. Then I tested them through the demo page and they were successfully recognized. They all had the "vv" prefix.
I use them to log into lastpass. I used them successfully for over a week, even on Sunday morning. Then on Sunday afternoon, I could no longer login to lastpass. I kept getting an error. I tried to re-register them with lastpass, but they would come back as invalid. So then I went back to the demo-test page, and none of them could be validated. This was true for all three yubikeys.
At the same time, some other yubikeys that I own were able to validate without a problem...even the ones with an old version of firmware 2.2.3 (that I also programmed for OTP myself, using the same method as described above).
The yubikeys were in my possession the entire time (two were in a box in a closet). I did not use them for anything other than lastpass login. Yet, about a week into the first use they all inexplicably failed. I reprogrammed them, and reregistered them, and now they work fine again. They are not "password protected". However, noone (including myself) did anything to them.
I can send you the configuration excel file for the three yubikeys from, 02/14/2014. I unfortunately did not copy the error explanation from the demo page.