The content you are loading, is it loading from a web page you control, or not?
The obvious difference between Mac, PC and iOS device is that the browser object used to load the web viewer is different on all three. Usually safari based in Mac, IE based in PC, and Safari mobile based in iOS.
Set a webviewer to: What browser am I using?
and see how the results change for each of the three platforms.
In FileMaker, you can setup different layouts for each of those three options, and redirect by using Get(SystemPlatform). That way you can at least size for what content is loading.
As for why you would get that error in iOS. Chances are the site you are trying to load has a mobile version and a desktop version.
When you visit the URL in safari on OSX, what does it say? Does it work in Safari on iOS but not in FM Go?