May i know do you have any other back up of the original catalog.?
Can you check that as well.
The easiest thing to do is to restore the most recent backup of your catalog file made before the installation of this plug-in.
If (and I'm not agreeing this happened yet) the installation of this plug-in did affect your catalog, it would not cause "backups could not be found anymore". That doesn't sound like anything a plug-in could do.
Also, this makes no sense: "ie. backups seemed to have been effective and working for some but all photos", Lightroom does not make backups of your photos.
Thanks for answering so quickly. I did indeed have further backups, and luckily I managed to load one. So I believe it is alright now. In fact I was just about to edit my initial post. However, I think I'll leave that post, as I still find this behaviour quite funny.
I could indeed restore the catalogue. I don't quite know how this could happen as well but as a matter of fact it did. I totally agree, if the plugin damaged the catalogue, it should rather throw an error message. But installing this plugin was the only thing I did between having a working catalogue and a damaged one.
In any case, it shows how important multiple backups are!