Trying to open Axure 8 RP file in Axure 9; but prompts to uninstall?

We have a couple of users that use Axure

Some of them are using Axure 8 on Macbook Pros
Some of them are using Axure 9 on Windows machines

One of our users was sending a AXure 8 file (created on the Mac version) to a user on the Windows version and although the file can be opened through the application itself if you try to open the file from file explorer it pops up with a prompting asking do you want to uninstall Axure 9. Is this a compatibility bug between the Mac and Windows versions of the application

Machines are running Windows 10 64 Bit 1709 and MacOS Mojave respectivley

This may be due to the way Windows handles default applications for file types. Try making sure the default for .rp files is Axure 9 (the preference is somewhere in the Windows control panels). It should then work fine from explorer, although Axure will of course warn you about the lack of backward compatibility if you save any changes.