On the latest.

13x ...rfaceReputationGuide/ReputationGuideSkin.lua:57: attempt to index global 'REP_ReputationDetailFrame' (a nil value)
[string "@ReputationGuide/ReputationGuideSkin.lua"]:57: in main chunk

Locals:
addonName = "ReputationGuide"
REP =

{
IsClassicPhaseFour = false
LIMIT_TYPE_Incr = 12
NEW_STANDING_COLOUR = "|cFF6060C0"
ShowDragonflightPhaseThree = false
AfterClassic = false
ShowClassicSOMPhaseFour = false
IsTheLastTitan = false
Expansions = {
}
IsClassicSOMPhaseFour = false
IsAlliance = false
IsClassicPhaseThree = false
IsCataPhaseFour = false
ShowLegionPhaseThree = false
ShowWotlkPhaseFour = false
IsTheWarWithin = false
ShowTBCPhaseTwo = false
IsCataPhaseThree = false
GameBuildVersion = 11504
IsWoDPhaseTwo = false
IsMoP = false
ShowClassicPhaseFive = true
ShowClassicSODPhaseFive = true
ShowBfAPhaseFour = false
IsClassic = true
ShowCataPhaseThree = false
LIMIT_TYPE_Mine = 3
AfterWotlk = false
IsClassicSODPhaseFive = true
IsLegionPhaseTwo = false
IsHeroic = false
IsTheWarWithinPhaseOne = false
IsMidnight = false
MaxFactionReputation = {
}
IsShadowLandsPhaseTwo = false
IsClassicSODPhaseTwo = false
LOWLIGHT_COLOUR = "|cFFFF3F3F"
LIMIT_TYPE_Alch = 6
ShowShadowLandsPhaseTwo = false
AfterLegion = false
IsClassicSOM = false
IsClassicSODPhaseFour = false
AfterShadowLands = false
IsWotlkPhaseOne = false
ShowWotlkPhaseThree = false
IsClassicPhaseSix = true
ShowBfAPhaseTwo = false
IsShadowLandsPhaseOne = false
IsLegionPhaseThree = false
ShowClassicSODPhaseThree = true
ShowMoPPhaseThree = false
Enchan = false
ShowDragonflightPhaseOne = false
LIMIT_TYPE_Arch = 14
IsShadowLandsPhaseThree = false
ShowTheWarWithinPhaseOne = false
BFFLabels = {
}
IsBfAPhaseThree = false
IsClassicSODPhaseOne = false
ShowWoDPhaseThree = false
NEW_REP_COLOUR = "|cFF7F7FFF"
FACTION_BAR_COLORS = {
}
ShowWotlkPhaseTwo = false
ShowClassicPhaseTwo = true
IsMoPPhaseFive = false
IsMoPPhaseOne = false
IsBfA = false
Alche = false
Mine = false
ShowTBCPhaseFive = false
ShowTheLastTitanPhaseOne = false
Engin = false
IsWoDPhaseOne = false
ShowMoPPhaseTwo = false
IsMoPPhaseFour = false
IsHorde = false
LIMIT_TYPE_Gather = 4
AfterCata = false
ShowClassicPhaseFour = true
IsDragonflightPhaseOne = false
HIGHLIGHT_COLOUR = "|cFF00FF00"
LIMIT_TYPE_Ench = 10
IsLegion = false
ShowTBCPhaseOne = false
LIMIT_TYPE_Aid = 13
AfterDragonflight = false
IsClassicSOMPhaseOne = false
Incrip = false
LIMIT_TYPE_Skin = 2
IsWotlkPhaseThree = false
Skin = false
QUEST_COLOUR = "|cFFC0FFC0"
ShowShadowLandsPhaseOne = false
LIMIT_TYPE_Engi = 5
AfterBfA = false
IsShadowLands = false
LIMIT_TYPE_Fish = 16
Factions = {
}
IsClassicSODPhaseThree = false
Leath = false
ShowMoPPhaseFive = false
ShowClassicSODPhaseTwo = true
IsClassicPhaseFive = false
IsWotlkPhaseFive = false
LIMIT_TYPE_Blac = 7
QUEST_ACTIVE_COLOUR = "|cFFFF7F7F"
IsCata = false
ShowClassicPhaseSix = true
Jewel = false
IsTBCPhaseTwo = false
IsCataPhaseTwo = false
ShowShadowLandsPhaseThree = false
AfterWoD = false
IsDeathKnight = false
IsBfAPhaseOne = false
IsTBCPhaseThree = false
Tailor = false
ShowTBCPhaseFour = false
IsMoPPhaseTwo = false
LIMIT_TYPE_Tail = 8
ShowWotlkPhaseOne = false
ShowClassicSOMPhaseThree = false
IsCataPhaseOne = false
IsClassicSOD = false
Arch = false
IsTBCPhaseFour = false
IsClassicSOMPhaseThree = false
ShowWotlkPhaseFive = false
IsWotlkPhaseTwo = false
ShowCataPhaseTwo = false
AfterTheWarWithin = false
MaxFactionLevel = {
}
ShowMidnightPhaseOne = false
ShowWoDPhaseFour = false
Cook = false
IsTBCPhaseFive = false
LIMIT_TYPE_Jewel = 11
IsDragonflightPhaseTwo = false
BAG_BANK_COLOUR = "|cFFFFFF7F"
Black = false
AfterTBC = false
IsBfAPhaseTwo = false
ShowLegionPhaseTwo = false
ShowTBCPhaseThree = false
ShowClassicPhaseOne = true
BAG_COLOUR = "|cFFC0FFC0"
ShowWoDPhaseTwo = false
ShowWoDPhaseOne = false
IsMidnightPhaseOne = false
AfterTheLastTitan = false
IsMoPPhaseThree = false
IsHuman = false
ShowClassicPhaseThree = true
IsWoDPhaseThree = false
ShowBfAPhaseOne = false
IsDragonflight = false
HasTradingPost = false
ShowBfAPhaseThree = false
ShowCataPhaseFour = false
LIMIT_TYPE_Leat = 9
IsTBCPhaseOne =

Oke "1.2.6-release" is being uploaded now, it should at least fix the error caused by the changes made in combination with ElvUI, I will however have to do more work to make it look good with ElvUI again, so I will keep this issue open for now till it's fixed again.

I downloaded ElvUI for SOD for personal testing.