Symptoms
Inspite of the site itself is published correct, you got the error:
Unable to connect to Web Presence Builder server. Please contact the server administrator.
In the tmp/sitebuilder.log
[EMERG] [System] /JsonApi/ Base_SyntaxException: PHP Warning : PDOStatement::execute(): MySQL server has gone away
file: /var/www/vhosts/4/100809/webspace/siteapps/530/htdocs/include/Zend/Db/Statement/Pdo.php
Cause
Timeout defined in the my.cnf
was exceeded.
Resolution
Increase wait-timeout
configuration directive in /etc/my.cnf
and restart MySQL server.