WowAce

Basic Repro Steps:
- Uninstall every addon except Shadowed Unit Frames

- Open up Edit Mode

- Move one of your action bars

Result:
- A lua error will appear on screen.

- Your edit mode settings will not be saved.

Message: Interface/FrameXML/EditModeSystemTemplates.lua:358: attempt to perform arithmetic on local 'left' (a nil value)
Time: Thu Nov 17 15:57:00 2022
Count: 2
Stack: Interface/FrameXML/EditModeSystemTemplates.lua:358: attempt to perform arithmetic on local 'left' (a nil value)
[string "@Interface/FrameXML/EditModeSystemTemplates.lua"]:358: in function `GetScaledSelectionSides'
[string "@Interface/FrameXML/EditModeSystemTemplates.lua"]:330: in function `IsVerticallyAlignedWithFrame'
[string "@Interface/FrameXML/EditModeSystemTemplates.lua"]:500: in function `GetFrameMagneticEligibility'
[string "@Interface/FrameXML/EditModeUtil.lua"]:123: in function `GetEligibleMagneticFrames'
[string "@Interface/FrameXML/EditModeUtil.lua"]:217: in function `FindMagneticFrames'
[string "@Interface/FrameXML/EditModeUtil.lua"]:279: in function `ApplyMagnetism'
[string "@Interface/FrameXML/EditModeSystemTemplates.lua"]:578: in function `OnDragStop'
[string "@Interface/FrameXML/EditModeSystemTemplates.lua"]:1832: in function <Interface/FrameXML/EditModeSystemTemplates.lua:1831>

I have this issue as well.

Same issue here. Was a pain to find which addon was causing it since the error lists the edit mode itself.