To use fop2 with modern browsers, you must use a modern fop2. The browsers you are testing were not available at the time fop 2.23 was released. So, either use browsers versions that were available by that time, o upgrade fop2 to the latest release.
The problem is not fop2, but the browsers that updated some protocols without backwards compatibility. FOP2 still supports all versions of the websockets protocol, but browsers only supports the latest implementations.
Best regards,
