AddIns.Add statement throwing an Internal Error 51 - vba

Question

I am trying to install addins programmatically (more precisely, it is automated version update on Workbook_Open event) but I have run into an issue with the AddIns.Add method, which just "does not work". I copy the desired adding into C:\Users\[username]\Documents\Addins and then feed the full filepath to AddIns.Add, however the addin is not added, as evidenced by the subsequent statement failing (subscript out of range, the name of the supposedly added addin does not exist).
During the install attempt, the execution simply runs through the AddIns.Add without any issue (except the result) but on stepping through, I am getting Internal error (Error 51). I have tried a number of ways to work around that, add Application.Wait before and after the AddIns.Add to make sure it has sufficient time, putting it into a Do While Loop statement to attempt multiple executions, but to no avail.
AddIns.Add Filename:=sInstallPath & sFile
AddIns(sAddinFullName).Installed = True
Btw this worked until yesterday, when I did a couple codes updates but not even remotely close to this area. I think I had some issues with this in past because the statement was envelopped by Application.Wait (Now + TimeValue("0:00:01")), which I think resolved probably a similar issue but I cannot recall that any more.
Edit: Adding a broader part of the code - a function that does the installation proper and on success, returns True.
Function InstallAddin(sFullPath, sAddinName) As Boolean
Dim oAddin As Object
Dim bAdded As Boolean
Dim i As Integer
Do Until bAdded = True Or i = 10
For Each oAddin In AddIns
If oAddin.Name = sAddinName Then
bAdded = True
Exit For
End If
Next oAddin
If bAdded = False Then
'Application.Wait (Now + TimeValue("0:00:01"))
AddIns.Add Filename:=sFullPath, CopyFile:=False
Debug.Print "Attempt " & i
'Application.Wait (Now + TimeValue("0:00:01"))
End If
i = i + 1
Loop
If bAdded = True Then
'disable events to prevent recurrence - installing addin counts as opening its workbook
Application.EnableEvents = False
AddIns(sAddinName).Installed = True
Application.EnableEvents = True
InstallAddin = True
End If
End Function
sFullPath : "C:\Users\Eleshar\Documents\Addins\MyAddin - v.0.25.xlam"
sAddinName : "MyAddin - v.0.25"
The "MyAddin - v.0.25.xlam" file is present in the installation path.
There is a piece of code elsewhere, which ensures that a regular WB is open during this event.
Edit 2: The full functionality of the macro is:
On opening the file by a user, offering self-install.
On opening the file by a user, checking for previous installed versions, offering self-installation (after which it removes the old versions, including itself).
On Workbook_Open, checking a Sharepoint repository for any new versions, offering to install the newest one available and removing any older versions including itself.
Edit 3: So I found an interesting thing... AddIns.Add does not seem to work when executed from the code (the addin does not get listed in Developer > Addins). However when I type the same exact statement into the immediate window during the execution, it works and then the addin can get installed...

Answer 1

Since you do not show all your used code, please try the next one. I am using it to auto install the add-ins I design:
Private Sub Workbook_Open()
Dim Name As String, tmp As Boolean, n As Boolean, Merk As String
Name = ThisWorkbook.BuiltinDocumentProperties(1) '(1)
On Error Resume Next
tmp = AddIns(Name).Installed
If Err.Number <> 0 Then
Err.Clear: On Error GoTo 0
If Workbooks.Count = 0 Then n = True
If n Then
Workbooks.Add
Merk = ActiveWorkbook.Name
End If
AddIns.Add FileName:=ThisWorkbook.FullName
AddIns(Name).Installed = True
If n Then Workbooks(Merk).Close False
End If
On Error GoTo 0
End Sub
'(1) it represents the Add-inn title. It can be set programmatically or manual in Properties - Details - Title. When add-in is not open!

Answer 2

So I did not really figure out the issue with AddIns.Add, however I worked around that but having the macro directly edit the Excel registry keys to install the add in.
Sub AddinInstall(sAddinName As String, ByVal sFullPath As String)
Dim oShell As Object: Set oShell = CreateObject("WScript.Shell")
Dim i As Integer: i = 0
Dim iIndex As Integer
Dim sRegKey As String: sRegKey = "HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Excel\Options\OPEN"
Dim sSZ As String
sFullPath = ChrW(34) & sFullPath & ChrW(34)
On Error Resume Next
Do 'loop through registry keys (non-existent key results in error, so errors must be disabled) to find if lower version is installed
i = i + 1
sSZ = ""
sSZ = oShell.RegRead(sRegKey & CStr(i))
If Len(sSZ) > 0 Then
If sSZ Like "*" & sAddinName & "*" Then
Debug.Print sSZ
iIndex = i 'get number at the end of registry key name
End If
End If
Loop Until Len(sSZ) = 0
If iIndex > 0 Then 'previous version installed - overwrite
oShell.RegWrite sRegKey & CStr(iIndex), sFullPath, "REG_SZ"
Else 'previous version not found, create new registry key
oShell.RegWrite sRegKey & CStr(i), sFullPath, "REG_SZ"
End If
On Error GoTo 0
End Sub