After using a UDF Excel Stops Working during save - vba

Question

I have set up a keyboard shortcut (ctrl+alt+c) That calculates the users selection pretty basic I grabbed off of here
Private Sub Workbook_Open()
Application.OnKey "^%c", "CalculateSelection"
End Sub
Sub CalculateSelection()
On Error GoTo NoSelection
If Not Selection Is Nothing Then
If TypeName(Application.Selection) = "Range" Then
Dim Rng As Range
Set Rng = Application.Selection
Rng.Calculate
End If
End If
Exit Sub
NoSelection:
MsgBox ("No Range Selected")
End Sub
This works perfectly fine on all other cell and i can save my work when its all completed.
If I use this function on a cell that calls an API the function works fine until you save, Microsoft stops working entirely and I have no idea why.
Does anyone know why this might be happening? would this be cause by some object not being destroyed or is my excel corrupt?
All the API function work fine I've had no errors with them until I used this sub on them.
This is the function that sends the data to the API. It has different functions that are called from the sheet to format the data to XML and check whether the data being passed is correct and fully calculated.
Private Function PostToApiXmlToXml(Data As String, Route As String)
'Initialise Document Object Model to hold the XML
'And the xmlHttp to handle Sending Data
Dim xmlhttp As MSXML2.ServerXMLHTTP60
Dim XMLDOM As MSXML2.DOMDocument60
Set xmlhttp = CreateObject("MSXML2.ServerXMLHTTP")
Set XMLDOM = New MSXML2.DOMDocument60
'Load entire Document before moving on
'Data is a string but must conatin opening and closing tags.
XMLDOM.async = False
XMLDOM.LoadXML (Data)
'Using the Http object
With xmlhttp
Call .setOption(2, SXH_SERVER_CERT_IGNORE_ALL_SERVER_ERRORS)
'Set up a POST to the Base Address + Route
.Open "POST", BaseAddress + Route, False
'Content-Type Header is requred to let the API Know What type of Data you are Sending
.setRequestHeader "Content-Type", "application/xml"
'Accept Header Lets the Api know we want to recieve JSON Data
.setRequestHeader "Accept", "application/json"
'Send the Dom as XML in the http object
.send (XMLDOM.Xml)
'Take in the Response as text
PostToApiXmlToXml = .responseText
End With
Set xmlhttp = Nothing
End Function
I can use the keyboard shortcut anywhere on the sheet and save with no problems. and i can use it on single cells with API calls and press ctrl + s and it saves fine. But if i select multiple and press ctrl + alt + c then save with ctrl + s it corrupts the save and restores to previous save doesn't let me open or view any of the problems with the corrupted version.