Apple Mail warning: Unable to verify message signature

After upgrading from Sierra to Mojave (macOS 10.14), Apple mail started to display the warning *Unable to verify message signature* above all signed and encrypted emails. When clicking on *Details*, it says: *The digital signature is incorrect. The message may have been tampered with or corrupted since signed by (sender's name).* We are using S/MIME certificates signed by a self-signed root CA, but I doubt that this is the problem. When checking the certificates in the Keychain app, they are reported to be valid and good. However, if in Apple Mail, I get the above message and click on *Show Certificate*, the root certificate is reported to be valid and trusted, but the S/MIME certificate is nonetheless invalid for no obvious reason. Note that some report similar problems for emails with attachments only.