[mythtv-users] blank mythweb page and error in log

[mythtv at howorth.org.uk]

When I go to http://localhost/mythweb/ it spends quite a few seconds
thinking and then displays a completely blank page.

If I look in /var/log/apache2/error_log is see:

[Wed Jul 25 11:25:53.278193 2018] [php7:warn] [pid 7905] [client
127.0.0.1:40748] PHP Warning:  Unknown: function '0' not found or
invalid function name in Unknown on line 0 

[Wed Jul 25 11:25:53.352956 2018] [php7:error] [pid 7905] [client
127.0.0.1:40748] PHP Fatal error:  Uncaught Error: Class 'MythBackend'
not found in /srv/www/htdocs/mythweb/includes/utils.php:56\nStack
trace:\n#0 /srv/www/htdocs/mythweb/includes/db_update.php(57):
setting('recommend_enabl...', NULL,
false)\n#1 /srv/www/htdocs/mythweb/includes/init.php(46):
require_once('/srv/www/htdocs...')\n#2 /srv/www/htdocs/mythweb/mythweb.php(20):
require_once('/srv/www/htdocs...')\n#3 {main}\n  thrown
in /srv/www/htdocs/mythweb/includes/utils.php on line 56

Does anybody have any idea what is going on, or ideas on how to fix it,
please?