Howard Miller

Not all web services are available through the Ajax service. Is that what you mean?

Moodle version?

What do you mean "refreshing"? What *exactly* happens?

You've given us very little to go on but if adding a plugin breaks your site, a good move is to simply delete it.

In this case, delete the directory /path/to/your/moodle/{public}/theme/boost_union

...and all its contents.

Can somebody explain this namespaced thing to me like I'm 5 because I really don't understand it.

I was under the impression that older plugins with Bootstrap 4 should "just work" for the moment, but I take it this is not the case. The migration document is not very clear. Well, not to me anyway.

 

EDIT: 

Is it as simple as data- attributes have gained a -bs- in the middle? My concern is that I don't want to have a different 4.5 and 5.1 version of the plugin if I can possibly avoid it. 

You seem to be using database sessions - don't. You have found one reason why not.