Ran into both of these errors in one of my plugins and so i wanted to share the fix with everyone.

PHP Fatal error:  Uncaught TypeError: Argument 1 passed to OW_ErrorManager::exceptionHandler() must be an instance of Exception, instance of Error given  

Line 235 in ow_core/error_manager.php

Since php 7.1

The fix is just remove the word Exception 

original code       (Exception $e)

new code        ($e)

For the error :  [] operator no supported for strings

It means that since php 7.1 we can no longer just use

$myarray[] = whatever we load it with

it has to be initialized because since php 7.1 php will no longer automatically assign $var[] as an array, you must tell php that it is an array first.

$myarray = array();  or   $myarray = [];

Then you can use it as $myarray[] = whatever we load it with

hope that helps...