On Retail, using v10.1-039:

Suddenly today when I logged in and tried to move anything in Edit Mode, EDE is throwing up an error, the relocated position is not registering, and there is no way to save the changed position.

243x FrameXML/EditModeSystemTemplates.lua:535: attempt to perform arithmetic on local 'left' (a nil value)
[string "@FrameXML/EditModeSystemTemplates.lua"]:535: in function GetScaledSelectionSides' [string "@FrameXML/EditModeSystemTemplates.lua"]:507: in function IsVerticallyAlignedWithFrame'
[string "@FrameXML/EditModeSystemTemplates.lua"]:725: in function GetFrameMagneticEligibility' [string "@FrameXML/EditModeUtil.lua"]:130: in function GetEligibleMagneticFrames'
[string "@FrameXML/EditModeUtil.lua"]:283: in function FindMagneticFrames' [string "@FrameXML/EditModeUtil.lua"]:367: in function GetMagneticFrameInfo'
[string "@FrameXML/EditModeManager.lua"]:1063: in function `RefreshSnapPreviewLines'
[string "@FrameXML/EditModeManager.lua"]:170: in function <FrameXML/EditModeManager.lua:168>

Locals:
self = EncounterBar {
0 =
UpdateMagnetismRegistration = defined @FrameXML/EditModeSystemTemplates.lua:731
GetSnapOffsets = defined @FrameXML/EditModeSystemTemplates.lua:609
systemInfo =

{
}
IsFrameAnchoredToMe = defined @FrameXML/EditModeSystemTemplates.lua:694
TrySetCompositeNumberSettingValue = defined @FrameXML/EditModeSystemTemplates.lua:212
downKeys = {
}
LayoutChildren = defined @SharedXML/LayoutFrame.lua:209
GetSelectionOffset = defined @FrameXML/EditModeSystemTemplates.lua:540
ResetToDefaultPosition = defined @FrameXML/EditModeSystemTemplates.lua:254
GetScaledSelectionSides = defined @FrameXML/EditModeSystemTemplates.lua:532
SelectSystem = defined =[C]:-1
GetFrameMagneticEligibility = defined @FrameXML/EditModeSystemTemplates.lua:714
CalculateFrameSize = defined @SharedXML/LayoutFrame.lua:165
AddLayoutChildren = defined @SharedXML/LayoutFrame.lua:55
IsBelowFrame = defined @FrameXML/EditModeSystemTemplates.lua:499
SetScaleBase = defined =[C]:-1
HasActiveChanges = defined @FrameXML/EditModeSystemTemplates.lua:381
GetScaledCenter = defined @FrameXML/EditModeSystemTemplates.lua:525
EMEanchorTo = UIParent {
}
DoesSettingDisplayValueEqual = defined @FrameXML/EditModeSystemTemplates.lua:445
UpdateDisplayInfoOptions = defined @FrameXML/EditModeSystemTemplates.lua:456
GetSystemName = defined @FrameXML/EditModeSystemTemplates.lua:822
ProcessMovementKey = defined @FrameXML/EditModeSystemTemplates.lua:39
GetSettingValueBool = defined @FrameXML/EditModeSystemTemplates.lua:436
HasCompositeNumberSetting = defined @FrameXML/EditModeSystemTemplates.lua:385
IsLayoutFrame = defined @SharedXML/LayoutFrame.lua:36
DoesSettingValueEqual = defined @FrameXML/EditModeSystemTemplates.lua:440
MarkDirty = defined @SharedXML/LayoutFrame.lua:99
ShouldResetSettingsDialogAnchors = defined @FrameXML/EditModeSystemTemplates.lua:168
EMEanchorPoint = "BOTTOMLEFT"
GetChildPadding = defined @SharedXML/LayoutFrame.lua:150
EMESystemID = 37
isManagedFrame = true
IsToTheLeftOfFrame = defined @FrameXML/EditModeSystemTemplates.lua:481
GetSettingsDialogAnchor = defined @FrameXML/EditModeSystemTemplates.lua:465
hasActiveChanges = false
OnSystemLoad = defined @FrameXML/EditModeSystemTemplates.lua:3
ClearFrameSnap = defined @FrameXML/EditModeSystemTemplates.lua:653
GetCombinedCenterOffset = defined @FrameXML/EditModeSystemTemplates.lua:596
SetupSettingsDialogAnchor = defined @FrameXML/EditModeSystemTemplates.lua:369
dirty = false
SetPointBase = defined =[C]:-1
UpdateSystem = defined @FrameXML/EditModeSystemTemplates.lua:330
isBottomManagedFrame = true
UpdateDirtySettings = defined @FrameXML/EditModeSystemTemplates.lua:189
isHighlighted = true
BreakSnappedFrames = defined @FrameXML/EditModeSystemTemplates.lua:636
PrepareForSave = defined @FrameXML/EditModeSystemTemplates.lua:97
snappedFrames = {
}
SetPointOverride = defined @FrameXML/EditModeSystemTemplates.lua:137
UpdateClampOffsets = defined @FrameXML/EditModeSystemTemplates.lua:148
ignoreFramePositionManager = true
IgnoreLayoutIndex = defined @SharedXML/LayoutFrame.lua:40
OnAnyEditModeSystemAnchorChanged = defined @FrameXML/EditModeSystemTemplates.lua:828
GetManagedFrameContainer

That one sounds familiar. I remember early on in Dragonflight people (who weren't using my addon) reporting that error. Does it still happen without my addon?

Its something to do with "Magnetism" malfunctioning. Thats the system that makes frames during Edit Mode stick to each other so they can be moved together. Its not well implemented by Blizzard, which is why I try to avoid using it as much as I can. Try just turning off the magnetism option.