For developers, see https://github.com/Stanzilla/WoWUIBugs/wiki/9.0.1-Consolidated-UI-Changes#backdrop-system-changes

1x FlashTaskBar\libs\DF\cooltip.lua:169: attempt to call method 'SetBackdrop' (a nil value)
[string "@FlashTaskBar\libs\DF\cooltip.lua"]:169: in function build_main_frame' [string "@FlashTaskBar\libs\DF\cooltip.lua"]:290: in function CreateCoolTip'
[string "@FlashTaskBar\libs\DF\cooltip.lua"]:3413: in main chunk

Locals:
self = GameCooltipFrame1 {
0 =
FlashAnimation = {
}
Stop = defined @FlashTaskBar\libs\DF\fw.lua:653
SetBorderColor = defined @FlashTaskBar\libs\DF\fw.lua:2294
SetLayerVisibility = defined @FlashTaskBar\libs\DF\fw.lua:2306
Borders =

{
}
SetBorderAlpha = defined @FlashTaskBar\libs\DF\fw.lua:2278
Flash = defined @FlashTaskBar\libs\DF\fw.lua:658
}
(*temporary) = nil
(*temporary) = GameCooltipFrame1 {
0 =
FlashAnimation = {
}
Stop = defined @FlashTaskBar\libs\DF\fw.lua:653
SetBorderColor = defined @FlashTaskBar\libs\DF\fw.lua:2294
SetLayerVisibility = defined @FlashTaskBar\libs\DF\fw.lua:2306
Borders = {
}
SetBorderAlpha = defined @FlashTaskBar\libs\DF\fw.lua:2278
Flash = defined @FlashTaskBar\libs\DF\fw.lua:658
}
(*temporary) = {
bgFile = "DialogFrame\UI-DialogBox-Background-Dark"
tileSize = 16
edgeFile = "Buttons\WHITE8X8"
tile = true
edgeSize = 1
insets = {
}
}
(*temporary) = "attempt to call method 'SetBackdrop' (a nil value)"
DF = {
DropDownCounter = 434238
ClassCache = {
}
GetAsianNumberSymbols = defined @FlashTaskBar\libs\DF\fw.lua:414
CreateAnts = defined @FlashTaskBar\libs\DF\fw.lua:2237
NewLabel = defined @FlashTaskBar\libs\DF\label.lua:258
PanelCounter = 434238
PictureNameCounter = 434238
GetTemplate = defined @FlashTaskBar\libs\DF\fw.lua:1627
ClientLanguage = "enUS"
SetHook = defined @FlashTaskBar\libs\DF\fw.lua:1674
GetCurrentSpec = defined @FlashTaskBar\libs\DF\fw.lua:2656
CheckPoints = defined @FlashTaskBar\libs\DF\fw.lua:713
GetRangeValue = defined @FlashTaskBar\libs\DF\math.lua:57
GetCooldownInfo = defined @FlashTaskBar\libs\DF\spells.lua:781
ScheduleTimer = defined @AnnounceRare\Libraries\Ace3\AceTimer-3.0\AceTimer-3.0.lua:94
NewTextEntry = defined @FlashTaskBar\libs\DF\textentry.lua:536
EncodeString = defined @FlashTaskBar\libs\DF\textentry.lua:1241
SortOrder3R = defined @FlashTaskBar\libs\DF\fw.lua:636
Msg = defined @FlashTaskBar\libs\DF\fw.lua:609
PrintVersion = defined @FlashTaskBar\libs\DF\fw.lua:140
CreateScrollBar = defined @FlashTaskBar\libs\DF\scrollbar.lua:7
CreateGlowOverlay = defined @FlashTaskBar\libs\DF\fw.lua:2192
UnitGroupRolesAssigned = defined @FlashTaskBar\libs\DF\fw.lua:53
ShowTutorialAlertFrame = defined @FlashTaskBar\libs\DF\fw.lua:1298
CooldownsExternals = {
}
GetSpecializationRole = defined @FlashTaskBar\libs\DF\fw.lua:89
SpecListByClass = {
}
FRAMELEVEL_BACKGROUND = 150
RunHooksForWidget = defined @FlashTaskBar\libs\DF\fw.lua:1656
IntegerToTimer = defined @FlashTaskBar\libs\DF\fw.lua:489
SortOrder1 = defined @FlashTaskBar\libs\DF\fw.lua:621
GetDefaultColorList = defined @FlashTaskBar\libs\DF\colors.lua:170
button_templates = {
}
NewSwitch = defined @FlashTaskBar\libs\DF\slider.lua:942
ReskinSlider = defined @FlashTaskBar\libs\DF\fw.lua:2523
GetBestFontForLanguage = defined @FlashTaskBar\libs\DF\fw.lua:1497
CreateColorTable = defined @FlashTA

For me it stopped throwing errors when I've used the DF fork from here https://github.com/Tercioo/Details-Framework/tree/shadowlands - basically get this Details! Framework version and replace inside the /libs/DF

Thanks that worked for me as well.