As you can see - went through first time - no errors. I believe we got it all as well. But - I was surprised at how long it took for what was, as you said, quite a small database. I think it is likely that what you have there is a timeout - at least that is the only likelihood I can come up with. All the data constructs look just fine.

The importer tool rally does need rewrite - it is just pretty low on the priority list at the moment. But it does try and do too much in each session if there are many, many topics in a forum. So I think that is the problem.

Do you have a local and up to date version of your website because that should get through it OK. Else forcing a much longer and temporary timeout in the server might do it but that might still be pushing it.

How high do you think I should bump the  timeout - its a self managed VPS, so I can modify almost anything.

So it is not a timeout issue. I gave PHP a 512MB memory limit, upped the WP_Memory in wp config to 512, change PHP timeout to 600. Dies with exact same error in the exact same place. When it goes to start the 3rd forum under the first grouping.

I had a thought I would test, in that this script hadn't been updated in a while, I changed my PHP default from 7.0 to 5.6. Cleared the database and tried again. No luck, same error in same place.

I seemed to have gotten it to work. Still running.  Its looks like the scripts doesn't like to read the database when it in use in the forum. I made a copy of the database in the same server, then ran the conversion. So far so good. PHP memory to 512, timeout to 6000 

Curious. My fingers are crossed.

PHP of 512 and 6000 second timeout let it complete. Couple of minor message skip, but not enough to be of any factor.....

Now that I know it will work, I get buy the rest of the plugins and hope to do a full conversion in a few weeks.

Thanks.

Apologies for the outdated approach but glad, of course, that it worked for you. Sadly - only so many hours in the day to do all the things we would like to do.