I tested MetaMax on my local server before moving my sites to PHP7.2+. It seems
that MetaMax does not work with PHP7.1, 7.2, 7.3. This is an error report:

[01-Apr-2020 16:55:50 Europe/Berlin] PHP Fatal error:  Uncaught Error: []
operator not supported for strings in
C:\openserver\OSPanel\domains\skyblue214\modules\MetaMax\action.default.php:113
Stack trace:
#0
C:\openserver\OSPanel\domains\skyblue214\lib\classes\class.CMSModule.php(1403):
include()
#1
C:\openserver\OSPanel\domains\skyblue214\lib\classes\class.CMSModule.php(1479):
CMSModule->DoAction('default', 'mb6cdc', Array, '1')
#2 C:\openserver\OSPanel\domains\skyblue214\lib\module.functions.php(106):
CMSModule->DoActionBase('default', 'mb6cdc', Array, '1',
Object(CMS_Smarty_Template))
#3
C:\openserver\OSPanel\domains\skyblue214\lib\classes\class.CMSModule.php(253):
cms_module_plugin(Array, Object(CMS_Smarty_Template))
#4
C:\openserver\OSPanel\domains\skyblue214\tmp\templates_c\3816413b4868e71aadce5ded6a8df7aecdce56d5_0.tpl_head.10.php(78):
CMSModule::function_plugin(Array, Object(CMS_Smarty_Template))
#5
C:\openserver\OSPanel\domains\skyblue214\lib\smarty\sysplugins\smarty_template_resource_base.php(128):
content_5e84a9761d8ef2_81923281(Object(CMS_Smarty_Templ in
C:\openserver\OSPanel\domains\skyblue214\modules\MetaMax\action.default.php on
line 113