Getting this error on a new page:
ReferenceError: bootstrap is not defined at https://mydomain.tld/js/all.js?2887:8:22344
I assume this is why the scroll back up arrow does not work.
The very same works on another page, that Iām hosting on the same shared web host.
Export options are identical.
Where/how to debug? 
Try turning off the combine all JS option in export options.
Yes, that did the trick. Thanks! 
+ Great job on Blocs 6, works really well, this was my first Blocs 6 project.
Just to understand the logic: Why would possibly combining all JS into one file cause such an issue?
If combing all the JS is breaking the javascript on the page, it basically means there is an issue with one of the JS files used in the site. These files can be added to projects manually, from custom Brics and of course Blocs adds some too.
If you can send me a copy of the exported site with combine all JS enabled, I can likely spot the problem.