dokuwiki\Exception\FatalException: Could not check compatibility between syntax_plugin_cellbg::handle($match, $state, $pos, dokuwiki\Parsing\Handler $handler) and dokuwiki\Extension\SyntaxPlugin::handle($match, $state, $pos, Doku_Handler $handler), because class dokuwiki\Parsing\Handler is not available

dokuwiki\Exception\FatalException: Could not check compatibility between syntax_plugin_cellbg::handle($match, $state, $pos, dokuwiki\Parsing\Handler $handler) and dokuwiki\Extension\SyntaxPlugin::handle($match, $state, $pos, Doku_Handler $handler), because class dokuwiki\Parsing\Handler is not available

An unforeseen error has occured. This is most likely a bug somewhere. It might be a problem in the cellbg plugin.

More info has been written to the DokuWiki error log.