With Softaculous I have installed Mbew, but once the program is called it always gives me the error "file not found" with /owm/operator in the URL.
Most probably you should ask Softaculous. Their service is not the official way to install Mibew Messenger, so we just unable to provide any support for it.
Obvious tip: check the error log of the web server, maybe there is something meaningful.
But one should understand that without any knowledge of your system and its configuration (and we have absolutely no clue about how Softaculous configured the installation) it's just impossible to give any advice.
Even scaling the PHP version I don't solve the problem.
Version 5.6 is no longer available.
Mibew Messenger works fine with PHP 7.x