Matteo, many thanks! That was it. The missing intermediate certificates https://moodle.org/mod/forum/discuss.php?d=326457#p1313013 !
There was a 'moodle.example.com.ca-bundle' file in the Zip-Archive I received from Comodo. I had to point a parameter SSLCertificateChainFile to that file and restart Apache.
Moodle for mobile
MDroid throwing java.security.cert.CertPathValidationException
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