-2147417848 automation error the object invoked has disconnected from its clients

Biggest module is 5547 lines of code (about 500-1000 of that will be commented code). And a sheet has about 600 lines of code. Two other sheets have less than 50 lines each.

Biggest module is 279KB when exported as a bas file.

Hi

I chopped up my module into 4 such that the biggest bas file is 61k.

I re-checked that I have Option Explicit at the top of all used modules.

But this still didn't help.

I have got following code in "ThisWorkbook". Could this be a problem, since the issue comes only when I move away from this file, to another file, while macro is running?
I commented this, but the problem persists.

Option Explicit

Private Sub Workbook_Activate()
On Error GoTo ErrHandler
   'Disable right click when you select this workbook
   Application.CommandBars("Ply").Enabled = False  
Exit Sub
ErrHandler:
    MsgBox "FATAL ERROR!!!" 
    End_Prog
End Sub

Private Sub Workbook_Deactivate()
On Error GoTo ErrHandler
   'Enable right click when you select other workbooks
   Application.CommandBars("Ply").Enabled = True
Exit Sub
ErrHandler:
    MsgBox "FATAL ERROR!!!" 
    End_Prog
End Sub