P: Possible problem with catalog backup zip files

Good to hear. On right clicking the .lrcat catalog file, invoke "Choose Default Program..." command, in the following dialog that comes up, select "Adobe Photoshop Lightroom" as the default program to open such type of file, make sure the checkbox at the bottom is checked to "Always use the selected program to open this kind of file". Then click ok. Then double clicking a .lcat file will launch Lightroom CC 2015 by default for you.

Simon, Steve - it's fixed!! I've fixed the manual opening of the catalog, so I can now open both the working and backup catalogs. I've taken everything that wasn't necessary out of the folder where the working catalog resides. For the first time since I moved to CC2015, the backup on exit routine from LR completes without stalling. That is a huge relief - I can't thank you enough for helping me fix this. It's been worrying me for months!

Simon - thanks for your suggestion. I've done the right-clicking, and get two options : LR 64-bit, which gives the "too new" message, and LR, which opens the catalog successfully. I'll see if the same is true with a backup catalog later today. It's a great relief to know that I can open LR.

Steve - many thanks for this. I've looked at the thread, and it look as if it could be the answer. However, I'm very nervous of registry changes - never in 25 years PC use. If I get it wrong, do I risk the catalog becoming non-functional? At present, LR is opening and running OK, but I can't open it from the catalog or any of the backups. Would uninstalling and reinstalling LR have any chance of working, if it is the registry problem?

There was a problem when LR CC came out, and some of the registry keys associated with LR for AutoPlay, at least, were still associated with LR 5.7. This was supposedly fixed with LR 6.1/2015.1 but maybe everything wasn't fixed.

If you know what you're doing try the registry change shown, here:
https://forums.adobe.com/thread/1825013

You can invoke the registry editor using Windows CMD+R (run) regedit

The "Lightroom Catalog.lcat" is your Lightroom CC 2015 catalog based on what your Catalog Settings dialog says.

However, double clicking the "Lightroom Catalog.lcat" to open the dialog might not be doing the right thing depends on your setup. You might have associated file type ".lcat" to open it in Lightroom 5. That is most likely what the "cannot open" message say, because it has been clearly indicated in the first screen capture that you are able to open it in Lightroom CC 2015.

Try do a right click on the "Lightroom Catalog.lcat" catalog and open it with the Lightroom CC 2015 from the context menu.

Sorry - got that wrong, the last modified catalog is just LR, not LR5. However, if I try to open LR by double clicking on this catalog, I get the "cannot open" message:

Next is the folder 2014-12-05 1296, which from the dates is the one containing the catalog from the last time I worked in LR, although it is showing it as an LR5 catalog: . I have no idea what the other folders are, and I'm puzzled by the set of CC2014 folders. I hope this makes some sense!

This is probably going to become incomprehensible pretty quickly. First screenshot is where LR says my catalog is: . Next screenshot is the Explorer view of that folder: . I'll pause at that stage and hit reply to see if this works and if the screenshots can be read before I go any deeper.

Just use the Add Image button at the lower right of the text area to include screenshots:



And your "current catalog" would be the one with the most recent modify date and also the one listed in LR / Prefs / Catalog Settings.