Event Espresso 4 is compatible with PHP 7. We know it is compatible because Eventsmart.com runs Event Espresso 4 and its servers have PHP 7. What’s probably happening is the host’s PHP 7 configuration has some type of bytecode (Opache or APC) caching activated, where the 5.6 configuration doesn’t have bytecode caching activated.
Event Espresso 4 may have issues with bytecode caching because some server configurations for bytecode caching may result in class EE_Declined_Registration_message_type
getting changed to something like class \xa0L\xdaor\x7f