SQL and other errors after Upgrading from 1.2.12

[Serpius]

Sorry for the massively delayed response - life interfered, interesting point about the table_prefix, might explain the issue, unfortunately it doesn't explain what to do about it...

Presumably, if you switched processes with the database handler, upgrades around that time would have taken that into account?  Would there be mileage in doing a 2 step upgrade, current ancient > some intermediate version > current?

Doubt it helped the first time round that I have about 17 modified templates, worth reverting them you think? or is that unrelated?

Wow... MyBB 1.2.xx!!

That's a massive upgrade. 

But, to answer your question... at the very least... if you can find the old MyBB versions... starting with 1.4.xx... do the conversions in smaller increments.

Then... 1.6xx, 1.8.xx... work your way up to 1.8.17. No guarantee that this method would work... in theory... this method should. 

However, that could be a time-consuming adventure and you've already have had an adventure or two from what I have read so far.

Now, that brings me to this question... if the website (the domain name and all of the other registered stuff that goes with it) is in your name, but it has not been active for years...

Why not start with a brand new slate? I kind of doubt that any of the old users will still be around, if at all.

In regards to the theme, most likely, if there are themes for the 1.2.xx series, those may not work with the 1.8.xx series due to many changes that have happened over the years.

Same goes for the plugins. Many old plugins no longer work due to incompatibilities and other issues, whether it be due to the series upgrade of MyBB or, a newer version of PHP and MySQL. 

So... save yourself the time and the hassle of attempting to retrieve data that may not even be feasible to use today, start fresh. 

Just my 2 cents...   

Oh... oh... please use PHP 7.xx.xx because MyBB 1.9.xx will require at least PHP 7.0.xx