Signtool fail (-2146434967/0x80100069)

I've been trying to automate signing files with a token smart card.

I wrote a short script that copies an exe file from elsewhere and signs it, in an infinite loop.

After a few hours of my script running successfully, the logging of the server that is executing this script shows the signtool failed with error (-2146434967/0x80100069).

I googled this and found that it means "The smart card has been removed, so that further communication is not possible."

The smart card was never removed, and I am clueless about the cause of this. I suspect it's something pre-defined in the windows registery. Help?

I set the token to be single sign-on so It's not supposed to ask a for a password after I enter it once until I log out of the computer.

Thanks.