[sylpheed:36648] GnuPG: key not found

[Jan]

Hi,

I am using Sylpheed 3.6.0 on Ubuntu 16.4, gpg (GnuPG) 1.4.20.

I have multiple email accounts, some of them have a key pair in gpg. Sylpheed
is configured with gnupg support. I can verify that the keys are there using 
 gpg --list-keys

When trying to send an encrypted email, Sylpheed does not find the private
key for the account, but I get the error message:
 no keys found for keyid "me at mail.com"

where me at mail.com is the mail address of the account.

Do you have any ideas why this is? How could I debug this?

Thank you!