Arbortext Editor 6.0 "File > Open" is not working

I was given a new work computer so we had to reinstall Arbortext Editor. We have v6.0 (Pine)

When I try to open an SGML file using "File > Open" or by clicking on the "open" folder icon I get the following errors:

• First attempt I get a window: "Attempted an unsupported operation." so I click OK.
• When I try it again with the application still running I get: "[A30363]Unrecognized command or macro: FileOpen" in the lower message bar section.

I was running Win 7 before but my new computer has Win 10. It was working fine before on the old computer. Also, if I double click on an SGML file it works fine - it launches the software and opens the file.

Any ideas? Any patches available?

Thanks.