Error appearing when accessing Matomo: Adapter '' is not valid


All of a sudden we started getting this message when we access Matomo:

Adapter ‘’ is not valid. Maybe check that your Matomo configuration files in config/*.ini.php are readable by the webserver.

I have already verified that the files are readable and the various server components are all working fine. Have also verified that the files are all fine with respect to virus/malware attacks.

Can someone throw some light on what might be wrong and are there any logs that can be checked?

Thank you.