Nitrokey Pro stopped working with Thunderbird 60.5.1

My Nitrokey Pro suddenly stopped working with Thunderbird 60.5.1. (under Linux Mint Cinnamon 19.1 with latest updates applied)
While in the past I was asked for my User Pins when I started Thunderbird, this does not happen any more. The Certificates are still registered (visible in Thunderbirds Security Tab),
but not accessible. Neither sending signed messages nor saving a draft is possible:

Unable to save your message as a draft.
Unable to sign message. Please check that the certificates specified in Mail & Newsgroups Account Settings for this mail account are valid and trusted for mail.

Starting the Nitrokey App also claims that Nitrokey Pro succesfully connected.

This is not a Nitrokey issue. It is just GPG and pinentry.

I heard of this issue multiply times, but I don’t know how to proper fix it. Seems to be a bug with pinentry and Thunderbird.
You can work around with the following: Get a GPG encrypted message in a file. Than open a terminal and execute gpg --decrypt message. Enter your PIN. Than go into Thunderbird and you should be able to sign and decrypt mails again as long as your key is unlocked.

I should have mentioned: I use S/MIME, not GPG - That is, my Certificates are issued by an agency.