To validate the OTP with the online Yubico validation server, it must be 44 characters.
We would appreciate if you can try generating OTPs consecutively on a notepad and check if all the OTPs generated are 44 characeters. If the OTPs are not 44 characters, please send us the following information:
1) The firmware version of the YubiKey. To check the firmware version, please visit the following link:
viewtopic.php?f=2&t=85&p=804&hilit=yubikey+version#p8042) Operating system details
3) Keyboard layout details