OnlyKey is sending an ASCII character code when you press a key. Different keyboard layouts interpret the character code differently. The keyboard layout set on the OS will need to match the keyboard layout set on OnlyKey. Here is a useful tool in figuring out what codes match up to what keys on your keyboard:
Same for me - have some passwords with a ^ as a special character, which I’m also not able to change - and even if I copy&paste the password to the OnlyKey UI, it does not use that key.
Changed the keyboard layouts (even if I have a Mac) to other layouts also, still up to no avail.
So the DUO is somehow useless for me, as it was bought just for not needing all that password manager back and forth…