I'm getting black squares in place of some frames when entering BGs today. Updated to latest build, still the same problem. Doing /reload fixes it, but I have to do it in every BG.

I just realized the whole frame is replaced by a castbar for some reason.

Well what are your raid cast bar settings?

Not enabled for raid. I'm only saying it's replaced by castbar because if I target myself and try to mount, I get a huge castbar instead of target frame.

It's still happening from time to time :(

Could you upload your config? "\World of Warcraft_classic_\WTF\Account<AccountName>\SavedVariables\LunaUnitFrames.lua"

https://pastebin.com/NJDWSUEC

The bug is not consistent though, on some days I get it in almost every match, on others I don't get it at all. Not sure what it correlates with.

Seems there's some error in borders:

Framestack:

The error you posted hints at RCLootCouncil causing issues. I have changed the corresponding code a bit. Try downloading https://github.com/Aviana/LunaUnitFrames/raw/master/modules/highlight.lua and https://github.com/Aviana/LunaUnitFrames/raw/master/modules/borders.lua and put them in your Luna "modules" folder overwriting the existing ones. Please tell me if this fixes it.

Isn't that's just error handler from RCLC? Thanks, I'll try it, though.