I took over for another person in owning our RoboHelp site in November, and ran into no issues until Mid-January. After I generated the output for the project, saved and closed the project, then reopened it the majority of my page links became broken (only 190/2,300 still work). I tried clearing cache, had saved a back-up of the project to our server, so I tried redownloading and deleting the one that I was working from (as I had made a minor update, easy to edit), and nothing worked.
I had the person who used to own the project attempt to open the project on their computer, by also redownloading my backup from the server and everything looks fine on their RoboHelp. The only differences between our RoboHelps that I can think of is that I had the 2019 version installed on my computer before I uninstalled the 2019 version and installed the 2017 version, and the fact that I am the one that made this update.
The only thing I am seeing is that, there is a trend towards the pages that work being part of the TOC, but even that is inconsistent.
Let me know if there is any further information that I can provide to give greater context to this issue.
When you uninstall, there is an option to save or remove preferences. If you did not remove the preferences, your old 2017 preferences would have been transferred to 2019 and were then picked up when you reinstalled 2017. I am not sure if that would have happened and can't think how that would give rise to this situation but it is one thought that came to mind.
Another is after uninstalling 2019, did you reboot before reinstalling 2017? It should not make a difference I'm struggling to think of anything else that would affect the installation beyond what I have covered.
Updates apart, I am thinking it is more likely to be a project issue so turning to the project. You say the previous owner downloaded a backup and it looks fine. Are you looking at the same backup? Have you downloaded again?