I downloaded sArena for tbc (arena addon) which uses the standard ui to create Arena frames.
Since Luna Unit Frames hides blizzard arena frames by default I had to untick the 'Arena' box in 'Hide Blizzard' for sArena to work. After that I also disabled Luna Unit Frames' own arena frames to not get duplicate arena frames.

After I had done the above I keep getting a LUA error every time I log in in or reload ui. This even occurs when I revert back to how it was before (hide blizzard arena frames and enable luna arena frames).

The only way for me to remove the Lua error is to disable the sArena addon.

Lua error:

Message: Interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:405: attempt to index field 'db' (a nil value)
Time: Thu Jun 17 16:51:17 2021
Count: 1
Stack: Interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:405: attempt to index field 'db' (a nil value)
[string "@interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua"]:405: in function `HideBlizzardFrames'
[string "@interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua"]:1507: in function <Interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1502>

Locals: self =

{
SetupAllHeaders = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1419
stateMonitor = {
}
MediaForced = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:268
ReloadSingleUnit = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1443
HideBlizzardFrames = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:404
InitializeUnit = defined @interface\AddOns\LunaUnitFrames\modules\layout.lua:496
AutoswitchProfileSetup = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1542
frameIndex = {
}
ReloadHeaderUnits = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1427
PlaceAllFrames = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1132
unitList = {
}
version = 4140
LoadMedia = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:250
LoadDefaults = defined @interface\AddOns\LunaUnitFrames\modules\defaults.lua:17
ResetColors = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:206
CorrectPosition = defined @interface\AddOns\LunaUnitFrames\modules\movers.lua:154
ReloadAll = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1471
Print = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:147
OnProfileDeleted = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:304
SetupHeader = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1394
L = {
}
IndicatorData = {
}
UpdateMovers = defined @interface\AddOns\LunaUnitFrames\modules\movers.lua:145
ApplySettings = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:668
OnLoad = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:219
fakeUnits = {
}
LoadoUFSettings = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:151
HeaderFrames = {
}
PlaceFrame = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1116
oUF = {
}
Reload = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1463
CreateConfig = defined @interface\AddOns\LunaUnitFrames\modules\options.lua:141
MediaRegistered = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:260
overrides = {
}
AutoswitchProfile = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:322
ProfilesChanged = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:272
SpawnUnits = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:1167
PlaceModules = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:982
ProfileReset = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:291
}
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index field 'db' (a nil value)"
handleFrame = defined @interface\AddOns\LunaUnitFrames\LunaUnitFrames.lua:361
active_hiddens = {
}