Collabora Online shows blank screen when opening document

Running NextCloud on VM with Apache.
Enabled both ā€˜Collabora Onlineā€™ & ā€˜Collabora Online - Built-in CODE Serverā€™ apps.
Setting for Collabora Online set to ā€˜Use the build-in CODEā€™.

Tried opening a .DOCX document but NextCloud shows a blank screen with (X) at the top-right corner, as if it canā€™t load the contents of the document and doesnā€™t want to show any toolbar.

(most installation guides for Collabora on NextCloud show 3 steps to enable Collabora, which I followed. Some recommend setting up a separate VM with docker for Collabora but I was hoping to use the Build-in option to save resources on my VM host)