What to do if you have this kind of error:
Warning: require_once(wp-content/plugins/tickera/includes/tcpdf/tcpdf.php): failed to open stream: No such file or directory in /wp-content/plugins/tickera/includes/tcpdf/examples/tcpdf_include.php on line
Fatal error: require_once(): Failed opening required '/wp-content/plugins/tickera/includes/tcpdf/tcpdf.php' (include_path='.:/usr/local/php56/pear') in /wp-content/plugins/tickera/includes/tcpdf/examples/tcpdf_include.php on line
You probably have an antivirus installed and it is falsely detecting this file as a threat.
Head over to the antivirus plugin settings and ignore the file in question (tcpdf_include.php ). Every antivirus plugin is different so look for an ignore option for your version of Antivirus. Then log into your FTP and navigate to /plugins/tickera/includes/tcpdf/examples/ you will probably see the name of the file changed to tcpdf_include.php.suspected. Just change it back to tcpdf_include.php after you have added it to ignore list.
The reason why this file is suspected is because it is used to generate a ticket file on your server so your system might see it as a threat.