Phase 5 of SoD has broken this addon, something to do with GetMouseover no longer working. Luckily the fix is simple:

\LunaUnitFrames\libs\oUF_Plugins\oUF_Highlight\oUF_Highlight.lua
\LunaUnitFrames\libs\oUF_Plugins\oUF_BorderHighlight\oUF_BorderHighlight.lua

Both of these files, look for lines that have "hasMouseover" and comment them out. Like this:

Addon works just fine after doing that in both of those files.

The GetMouseFocus function has been replaced by GetMouseFoci.

https://warcraft.wiki.gg/wiki/Patch_11.0.0/API_changes

I got it working by swapping
GetMouseFocus()
to
GetMouseFoci()[1]

I think the idea is that the mouse might be on top of a lot of things, and the element directly beneath it is item 1, then the thing a layer below is item 2, and so on.

your mileage may vary.

The above worked for me. Thanks Tegas!

Steps to fix:

1. Open file: "LunaUnitFrames\libs\oUF_Plugins\oUF_BorderHighlight\oUF_BorderHighlight.lua"
2. On line 79 replace GetMouseFocus() with GetMouseFoci()[1]
3. Open file: "LunaUnitFrames\libs\oUF_Plugins\oUF_Highlight\oUF_Highlight.lua"
4. On line 66 replace GetMouseFocus() with GetMouseFoci()[1]
5. Save, reload, etc.