Importing catalogue fails due to unknown error, images still imported but without keywords

[LifeInRaster]

I have two large Lighroom Classic 9.1 catalogues which I wanted to merge into one. Both catalogues have around 22,000 photos, most of which have keywords, captions etc. Note, the keywords in both catalogues have the same hierarchy and structure.

I started the 'import from another catalogue' and let it run for a few hours. When I returned there was an error message stating that the import had failed due to an unknown error. Very helpful indeed. 'Most' of the photos from the catalogue I was importing from are now present in the new catalogue (which photos weren't imported? yep, no idea). I noticed that nearly every imported photo has been stripped of keywords. Strangely, the keyword list was updated with keywords completely out of the existing hierarchy and structure, and with no photos associated with those keywords. Reading metadata from photos didn't help either as the photos themselves had been cleared. XMP sidecars are probably wiped too.

So, I will delete all the photos recently imported, fix up the keyword list and try agin. Question for the community is, how can I merge my two large catalogues?

[LifeInRaster]

Might it have something to do with the Replace option for existing photos? I had set it to "nothing" as that made the most sense to me. But when after I deleted all the imported photos, I ran an import again on a small folder of photos, this time setting the Replace option to "Metadata, develop settings and negative files". The folder of photos imported into the new Catalogue with all its metadata present. 

UPDATE

Changing the Replace option did not work either. The result is the same as my first import attempt.

[Pinner Photographer]

I have hit this same problem today. After importing several catalogs into my new master catalog the next one failed with the amaizingly useless eror message.

@Adobe. Surely this is primarily a transfer of data from one database to another. It is as if the code to run the import has some issues with the latest update to the catalog structure. If you need some data to test this there are plenty of examples out there including mine. If you have known defects in this code and you are working on a fix just let us know so that we can stop wasting our time and get on with our photography.

@gtk-gigapix I exported my collections to catalogs one by one from the rogue source and they imported to the new catalog without any issues and with the right number of images.

PS To fix one of the previous import errors I deleted 15 collections in the Master and went back to the rogue catalog where I put the original 15 collections into a collection set. I then exported the collectin set as a catalog. That also imported without issue back into the Master. This is potential time savour for me