Custom taxonomy 404 on page 2 +

Hi there i have been at this most of the day and got nowhere – well somewhere…
The pagination links are showing on the taxonomy archive as pretty ones. Ok i have /%category%/%postname%/ set for the site.

https://domain.net/custom-tax/a-term/page/2/
These go to 404 when clicked

If i change the link at the end to a query string like this
https://domain.net/custom-tax/a-term/?page=2/
It loads page 2 as expected.

If i go to permalinks > settings and change to Plain it doesn’t work though.

So is there a way i can influence pagenavi to use the query string for this taxonomy loop or have i found a bug?

Hoping against hope, fingers crossed…

The page I need help with: [log in to see the link]