DigiDoc4 Client message: updating the list of trusted certificates was unsuccessful

Did you see the following error message when starting DigiDoc4?

Updating the trusted list of certificates used for checking the validity of digital signatures was unsuccessful.  Please check that your computer is connected to the Internet and you have the latest ID-software version installed. A dated list will be used for signature verification.

This error message implies that updating the trusted list necessary for checking and signing signatures was not successful. Use of the dated trusted list is allowed; however, you may not be able to check the validity of some signatures.

There are several reasons that such a situation may arise: