Blank modal rendering on Adobe Document Cloud SDK viewer

I have a viewer on my website which is supose to show content from a PDF file. It loads perfectly sometimes, but most of the times it displayes a blank modal content. It does triggers an event APP_RENDERING_START but then no other messages can be seeing, neither the content is display inside the pop up modal when it opens. 

Any suggestions on what the problem might be as i didn't find many answers on web?

Thank you!