Simultaneous Phalcon + WAMPP request problem

I'm creating an application that will run a SPA on the front end with VueJS and on the backend a webservice with Phalcon. I'm having a problem when I make two asynchronous calls simultaneously to the API (located on my test machine) I get the following error on the second call:

<br />
<b>Fatal error</b>:  Uncaught Error: Access to undeclared static property: 
Phalcon\Di::$_default in D:\wamp64\www\web-service\public_html\index.php:19
Stack trace:
#0 [internal function]: Phalcon\Di-&gt;__construct()
#1 D:\wamp64\www\web-service\public_html\index.php(19): 
Phalcon\Di\FactoryDefault-&gt;__construct()
#2 {main}

Next Error: Access to undeclared static property: Phalcon\Di::$_default in 
D:\wamp64\www\web-service\public_html\index.php:19
Stack trace:
#0 [internal function]: Phalcon\Di-&gt;__construct()
#1 D:\wamp64\www\web-service\public_html\index.php(19): 
Phalcon\Di\FactoryDefault-&gt;__construct()
#2 {main}
  thrown in <b>D:\wamp64\www\web-service\public_html\index.php</b> on 
line <b>19</b><br />

If I run each call one after another, both work correctly.