[JKT]

It is a javascript issue - some others have reported problems with Print buttons on pages (they typically use javascript for their output). If it works in the current official release of Safari, then there has been a regression in the Javascript engine since that release that needs to be or has been fixed in WebKit. Potentially it should work when OmniWeb upgrades to an even newer version of WebKit than the one it is using with the 5.6 release (assuming they manage to do so). In the meantime, you could test to see if it is due to the version of WebKit being used and if it is corrected in a more recent nightly build.

N.B. send a bug report to OmniGroup using the Send Feedback option from the help menu so that they are made aware of your problem.