WowAce

What version of Grid2  are you using (Version is displayed in General/About Tab) ?
2.9.12

What game client version, classic or retail, windows or mac and language are you using ?
Midnight Beta 

12.0.0.64339

What steps will reproduce the problem?

On Login

  
Did you try having Grid2 as the only enabled addon and everything else disabled?
Yes

Was it working in a previous version? If yes, which was the last good one?
TWW Season 3 
11.2.5.642710

Bugsack first report (second below):

254x Grid2/modules/StatusHealth.lua:390: attempt to perform arithmetic on a secret value
[Grid2/modules/StatusHealth.lua]:390: in function 'IsActive'
[Grid2/GridIndicator.lua]:159: in function 'GetCurrentStatus'
[Grid2/GridIndicator.lua]:103: in function 'Update'
[Grid2/GridFrame.lua]:195: in function 'UpdateIndicators'
[Grid2/GridFrame.lua]:89: in function <Grid2/GridFrame.lua:81>
[C]: in function 'SetAttribute'
[Blizzard_RestrictedAddOnEnvironment/SecureGroupHeaders.lua]:210: in function <...rd_RestrictedAddOnEnvironment/SecureGroupHeaders.lua:130>
[Blizzard_RestrictedAddOnEnvironment/SecureGroupHeaders.lua]:495: in function <...rd_RestrictedAddOnEnvironment/SecureGroupHeaders.lua:394>
[C]: in function 'Show'
[Grid2/GridLayout.lua]:742: in function 'ForceFramesCreation'
[Grid2/GridLayout.lua]:729: in function 'FixHeaderAttributes'
[Grid2/GridLayout.lua]:602: in function 'AddHeader'
...[C]: ?
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:19: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:15>
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:54: in function 'SendMessage'
[Grid2/GridRoster.lua]:406: in function 'GroupChanged'
[Grid2/GridRoster.lua]:338: in function '?'
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:109: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:109>
[C]: ?
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:19: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:15>
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:54: in function 'Fire'
[Ace3/AceEvent-3.0-4/AceEvent-3.0.lua]:120: in function <Ace3/AceEvent-3.0/AceEvent-3.0.lua:119>

Locals:
self = <table> {
indicators = <table> {
}
enabled = true
name = "health-low"
priorities = <table> {
}
dbx = <table> {
}
}
unit = "player"
m = 136660
(*temporary) = <no value>
(*temporary) = "player"
(*temporary) = "attempt to perform arithmetic on a secret value"
healthlow_threshold = 0.400000

second report: 

46x Grid2/modules/StatusHealth.lua:528: attempt to perform arithmetic on a secret value
[Grid2/modules/StatusHealth.lua]:528: in function 'GetPercent'
[Grid2/modules/StatusHealth.lua]:490: in function 'IsActive'
[Grid2/GridIndicator.lua]:159: in function 'GetCurrentStatus'
[Grid2/GridIndicator.lua]:103: in function 'Update'
[Grid2/GridStatus.lua]:74: in function 'UpdateIndicators'
[Grid2/modules/StatusColor.lua]:35: in function '?'
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:109: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:109>
[C]: ?
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:19: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:15>
[Ace3/CallbackHandler-1.0-8/CallbackHandler-1.0.lua]:54: in function 'Fire'
[Ace3/AceEvent-3.0-4/AceEvent-3.0.lua]:120: in function <Ace3/AceEvent-3.0/AceEvent-3.0.lua:119>

Locals:
self = <table> {
indicators = <table> {
}
priorities = <table> {
}
dbx = <table> {
}
name = "health-deficit"
enabled = true
}
unit = "player"
m = 136660
(*temporary) = <no value>
(*temporary) = "player"
(*temporary) = "attempt to perform arithmetic on a secret value"

Grid2 cannot work on midnight due to blizzard new restrictions. 

Im considering rewriting most of the statuses to work on midnight, but im not sure if it is worth because the new restrictions are so severe that it is  impossible to even replicate the basic functionality of blizzard raid frames: 90% of statuses and indicators provided by grid2 cannot work on midnight.