My understanding is it's a difference between HTML/ XML / XHTML.
In HTML, in the case of what is being pasted,
br is fine, but with the parser expecting XHTML, unclosed tags are not acceptable so all
br should really be
br / and that's likely why it's breaking.
The error message itself points to this, it's expecting either
br ...
/br or in the case of a break
br / which would be "more correct" if that makes sense!
I'm not an html / xml / xhtml "expert" but if you google "what's the difference between
br and
br /" I think you'll find it's talked about plenty.