Blank custom OmniCore app (on server)

Hi, I would like to be able to run an app previously developed for OmniCore on Windows. The original app created some .dat files on the controller. I want to do the same in my app, but I want to create them on my computer locally. I've already created a working server that is able to handle the file services via get and post requests. The issue is that once I've done it, after starting up the server I am not able to view the interface of the app anymore (it used to display correctly before my alterations). It seems that the program reads the index.html file, because even though the screen is blank, there is a gray square in the bottom left corner of the screen (just like in OmniCore apps). Does anyone have any idea what might be causing such an issue? I did not edit any of the files responsible for the display of the app.

I'm sorry if I described the issue not clearly enough, I am quite new to app development and do not know all the terminology yet :#