First, I'm assuming what you want to do is just a one-off. I.e., that you have an ontology that was developed in 4.3 and you don't want to redo it.
If that's not the case, if you want to keep using 4.3 because it has support for some tool or feature that you like,
my advice is to not do that. Even if you get this problem figured out there have just been so many changes to Protege, Java,
and the operating systems that support it since 4.3 that using 4.3 is very likely to be more trouble than it's worth
no matter what the reason for still wanting to use it.
If it is a one-off for what it's worth here's my suggestion: it looks like you are getting a UI error. I don't think I ever used 4.3 but there were
probably some features in 4.3 that are no longer supported in the current version. For example, perhaps there was a plugin that is no longer supported
or perhaps there was a feature like adding facets to properties. If you can still get 4.3 to work I would look through the ontology for anything like that
and delete it then save the OWL file again and try loading that.
Also, I would try saving in the Machester syntax. I know you tried that already but after saving I would look through the file manually and see if you can
spot anything that looks like it's not OWL 2.0 compatible or that it requires some capability or plugin no longer supported. I suggest doing that in Manchester syntax because in my experience it is the most readable of all of them.
That's probably not much help, perhaps others who have more experience with the older version or more knowledge of the internals of Protege