This was a very weird issue. The only thing I can think of was that something crashed the addon or something I have no idea how it is possible reading the code back. Perhaps the addon goes away, but we don't unregister UNIT_AURA?

26x BigDebuffs\BigDebuffs-v10.7.lua:330: attempt to index local 'anchors' (a nil value) [string "@BigDebuffs\BigDebuffs-v10.7.lua"]:330: in function func'
[string "@BigDebuffs\BigDebuffs-v10.7.lua"]:737: in function AttachUnitFrame' [string "@BigDebuffs\BigDebuffs-v10.7.lua"]:1819: in function UNIT_AURA'
[string "@BigDebuffs\BigDebuffs-v10.7.lua"]:705: in function <BigDebuffs\BigDebuffs.lua:705>

Locals:
anchor = "ElvUF_Player"
anchors = niln
unit = nil
(*temporary) = defined =[C]:-1
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index local 'anchors' (a nil value)"`