If curious, my site is https://wiki.kopacko.us. I upgraded from Greebo to Hogfather and now see a blank site. Below is the most recent error log:
Wed Sep 16 18:40:31.663412 2020] [php7:error] [pid 1644] [client <IPADDR RETRACTED>:60675] PHP Fatal error: Declaration of syntax_plugin_diagram_main::handle($match, $state, $pos, &$handler) must be compatible with dokuwiki\\Extension\\SyntaxPlugin::handle($match, $state, $pos, Doku_Handler $handler) in /var/www/html/dokuwiki/lib/plugins/diagram/syntax/main.php on line 0
[Wed Sep 16 18:48:41.494902 2020] [php7:error] [pid 2113] [client <IPADDR RETRACTED>:45882] PHP Fatal error: Declaration of syntax_plugin_diagram_main::handle($match, $state, $pos, &$handler) must be compatible with dokuwiki\\Extension\\SyntaxPlugin::handle($match, $state, $pos, Doku_Handler $handler) in /var/www/html/dokuwiki/lib/plugins/diagram/syntax/main.php on line 0
[Wed Sep 16 18:53:02.333198 2020] [mpm_prefork:notice] [pid 1551] AH00171: Graceful restart requested, doing restart
[Wed Sep 16 18:53:02.404393 2020] [mpm_prefork:notice] [pid 1551] AH00163: Apache/2.4.29 (Ubuntu) mod_jk/1.2.43 OpenSSL/1.1.1g configured -- resuming normal operations
[Wed Sep 16 18:53:02.404404 2020] [core:notice] [pid 1551] AH00094: Command line: '/usr/sbin/apache2'
[Wed Sep 16 18:53:10.077000 2020] [mpm_prefork:notice] [pid 1551] AH00171: Graceful restart requested, doing restart
[Wed Sep 16 18:53:10.204175 2020] [mpm_prefork:notice] [pid 1551] AH00163: Apache/2.4.29 (Ubuntu) mod_jk/1.2.43 OpenSSL/1.1.1g configured -- resuming normal operations
[Wed Sep 16 18:53:10.204187 2020] [core:notice] [pid 1551] AH00094: Command line: '/usr/sbin/apache2'
[Wed Sep 16 19:09:25.021135 2020] [php7:error] [pid 2775] [client <IPADDR RETRACTED>:62369] PHP Fatal error: Declaration of syntax_plugin_diagram_main::handle($match, $state, $pos, &$handler) must be compatible with dokuwiki\\Extension\\SyntaxPlugin::handle($match, $state, $pos, Doku_Handler $handler) in /var/www/html/dokuwiki/lib/plugins/diagram/syntax/main.php on line 0
Any suggestions on where to start?