Can you post your project for us to look at? Zip up everything in your project folder and post it to a file sharing site like Adobe Creative Cloud, Dropbox, or Google Drive.
Sorry for the late reply, I did find out what the problem was by now.
For some reason, nothing is displayed in the browser if the FILEPATH contains '_728x90'. Not even just the filename, I mean the entire filepath.
I figured it out when remaking the banner for the 4th time and testing in the browser after every step (but not saving, it was the 4th time I was redoing it..). It always worked and I was glad I finally made it to the end. Then I saved the file under a useful name and then it stopped working again. So I renamed it and found out that that was the problem.
I then figured, well if I cant put the resolution in the filename I'll rename the project folder to include it. It broke again.
I have to add: My other banner which is 338x280 has no issues at all. I can put '_338x280' in the filename without any issues. However '_728x90' breaks it.
Figured I should let the Adobe / the internet know. This problem drove me near mad, I hope Adobe can fix this soon and in the meanwhile people could find the problem here.
Glad you got it to work. I'll look into _728x90 - was it the name of the composition?