Unable to sign or decrypt with card

[Philip Jackson]

gpg (GnuPG) 2.1.11  in UbuntuStudio 16.04 LTS
libgcrypt 1.6.5

At the end of April, I made a detached signature of a file that I was
distributing. Today I updated that file and tried to make another
detached signature. The operation failed with a not very informative
error message :
gpg: signing failed: Operation cancelled

I checked the card-status and it seemed normal so I encrypted a test
file and that was ok. The PIN is certainly correct.

Then I tried to decrypt the test file and the operation failed with a
better message :

gpg: public key decryption failed: Operation cancelled
gpg: decryption failed: No secret key

So it looks like my secret key has vanished from sight and contact of
gpg in the past 4 months although I have changed strictly nothing in my
setup in that period - except regular UbuntuStudio security updates and
I don't recall having seen and gpg stuff go through.

gpg2 -K does list my secret key -

Suggestions as to how to check and correct this situation would be
appreciated.

Philip