Grid2 unit frames show up as black. Player character is solid black, while adding additional players to the party shows them with varying levels of opagueness (similar to when you click on the magnifying glass under layout to preview what layouts look like).

Debug mode messages are below:
09:09:43 Grid2: DEBUG Grid2 GroupChanged PLAYER_ENTERING_WORLD Kalimdor 1 nil other 1 => solo none 1
09:09:43 Grid2: DEBUG Grid2_Grid2Frame updated Grid2LayoutHeader1UnitButton1 unit player player <= nil
09:09:43 Grid2: DEBUG Grid2_Grid2Frame removed Grid2LayoutHeader1UnitButton1 unit player
09:09:43 Grid2: DEBUG Grid2_Grid2Frame updated Grid2LayoutHeader1UnitButton1 unit player player <= nil

Tried with different combinations of "Hide Blizzard Raid Frames" and "Use Blizzard Unit Frames". Unchecking "Use Blizzard Unit Frames" just makes all Grid2 frames disappear.

Also tried with a reset, default profile, but issue still persists.

This is the Lua Error that's being displayed:

Message: Interface\AddOns\Grid2\GridUtils.lua:194: attempt to call method 'SetBackdrop' (a nil value)
Time: Wed May 19 09:31:15 2021
Count: 9
Stack: Interface\AddOns\Grid2\GridUtils.lua:194: attempt to call method 'SetBackdrop' (a nil value)
[string "@interface\FrameXML\RestrictedFrames.lua"]:653: in function <Interface\FrameXML\RestrictedFrames.lua:651>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "@interface\FrameXML\RestrictedFrames.lua"]:657: in function <Interface\FrameXML\RestrictedFrames.lua:656>
[string "@interface\FrameXML\RestrictedFrames.lua"]:795: in function CallMethod' [string " self:SetAttribute("*type1", "target") self:SetAttribute("*type2", "togglemenu") self:SetAttribute("useparent-toggleForVehicle", true) self:SetAttribute("useparent-allowVehicleTarget", true) self:SetAttribute("useparent-unitsuffix", true) local header = self:GetParent() local clickcast = header:GetFrameRef("clickcast_header") if clickcast then clickcast:SetAttribute("clickcast_button", self) clickcast:RunAttribute("clickcast_register") end header:CallMethod("initialConfigFunction", self:GetName()) "]:12: in function <[string " self:SetAttribute("*type1", "target")..."]:1> [string "=(tail call)"]: ? [string "=[C]"]: ? [string "@Interface\FrameXML\RestrictedExecution.lua"]:484: in function <Interface\FrameXML\RestrictedExecution.lua:447> [string "@Interface\FrameXML\SecureGroupHeaders.lua"]:116: in function <Interface\FrameXML\SecureGroupHeaders.lua:110> [string "@Interface\FrameXML\SecureGroupHeaders.lua"]:166: in function <Interface\FrameXML\SecureGroupHeaders.lua:123> [string "@Interface\FrameXML\SecureGroupHeaders.lua"]:488: in function <Interface\FrameXML\SecureGroupHeaders.lua:387> ... [string "=[C]"]: ? [string "@Interface\AddOns\AtlasLootClassic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:29: in function <...sic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:25> [string "@Interface\AddOns\AtlasLootClassic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:64: in function SendMessage'
[string "@interface\AddOns\Grid2\GridRoster.lua"]:292: in function GroupChanged' [string "@Interface\AddOns\Grid2\GridRoster.lua"]:240: in function ?'
[string "@interface\AddOns\AtlasLootClassic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:119: in function <...sic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:119>
[string "=[C]"]: ?
[string "@interface\AddOns\AtlasLootClassic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:29: in function <...sic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua:25>
[string "@interface\AddOns\AtlasLootClassic\Libs\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:64: in function `Fire'
[string "@interface\AddOns\Bagnon\common\Wildpants\libs\AceEvent-3.0\AceEvent-3.0.lua"]:120: in function <...\common\Wildpants\libs\AceEvent-3.0\AceEvent-3.0.lua:119>

Locals:

It seems an incorrect version of Grid2 is installed, working version for burning crusade is 2.0.10-bcc:
https://www.wowace.com/projects/grid2/files/3313057
Try reinstalling the addon.

Forget previous comment, It's a bug in icons indicator, a workaround:
Goto DispellableDebuffsMultiple Indicator configuration , layout Tab.
Set Border Size to Zero.
I will update the addon to fix this issue in a few hours.

Version 2.0.11 should fix the issue:
https://www.wowace.com/projects/grid2/files/3315419