Hi Nick,
Yes I did, after a lot of time not getting any help here, I eventually found the solution.
So if you are adding your own .htaccess marker in Akeeba Admin Tools - you need to add the following exceptions.
Look for:
'Allow direct access, including .php files, to these directories:'
Then add these two new items:
administrator/components/com_sppagebuilder
components/com_sppagebuilder
https://prnt.sc/1y3i1li
This works for me, if it does for you also - perhaps Joomshaper can add this to the documentation somewhere.
Hope that helps
Britt