via Bugsack

91x AceLocale-3.0-6: ElvUI_SystemDatatext: Missing entry for 'System (Improved)'
[string "@ElvUI_SystemDatatext\ElvUI_SystemDatatext-2.2.6.lua"]:549: in main chunk

I disabled all addons but ElvUI, ElvUI_options and SystemDataText and the error persisted.

Fix by editing ElvUI_SystemDatatext\ElvUI_SystemDatatext.lua and edit line 549.

Change from...

DT:RegisterDatatext("System (Improved)", nil, {"PLAYER_ENTERING_WORLD"}, OnEvent, OnUpdate, OnClick, OnEnter, OnLeave, L["System (Improved)"])

to...
DT:RegisterDatatext("System (Improved)", nil, {"PLAYER_ENTERING_WORLD"}, OnEvent, OnUpdate, OnClick, OnEnter, OnLeave)

Easy fix, thank you very much!