Error 52 Bad File Name when using DIR / saving PDF to SharePoint / OneDrive

I created a file for a client that has (among other things) macros to save copies of various sheets as PDF files. The PDFs are saved in a specified folder that is under the folder containing the Excel file.
Everything worked fine until they apparently moved the Excel file to a SharePoint folder, and are accessing it through OneDrive. Now when they try to save the PDF, they get a runtime error 52 (Bad file name or number).
The code checks to see whether the standard filename already exists, and if so, it adds a sequential "Ver00" to the filename. Curiously, I have an error handler in place should an error occur during the filesave operation, but the macro is NOT reaching that error handler. So I'm thinking it's tripping the error 52 when running the Dir command. But why would this crash when the file is on SharePoint, and not when it's on a local drive? I have no experience with SharePoint / OneDrive so I am clueless on how to help. Any suggestions would be appreciated!
By the way, neither the Subdirectory nor the RefText contain any prohibited characters.
The code is:

Sub CreateSubTrans_PDFFile()
' Generates a PDF file of the active SubTrans sheet

    Dim CheckName                   As String
    Dim counter                     As Integer
    Dim FilePathAndName             As String
    Dim RefText                     As String
    Dim Response                    As Integer
    Dim Subdirectory                As String
    
    Subdirectory = [ProjectInfo_FileLocationTransmittals]
    
' Limit REF to 15 characters
    RefText = Left([SubTrans_Reference], 15)
    
' Export the print area as a PDF file
    FilePathAndName = ThisWorkbook.Path & Subdirectory & "\S-" & Format([SubTrans_TransmittalNumber], "00") & "-" & RefText & "-" & Format(Now(), "m-d-yy")
    CheckName = FilePathAndName & ".pdf"
    counter = 0
    If Dir(CheckName) <> "" Then      'File already exists
        counter = 2
        CheckName = FilePathAndName & "-Ver" & Format(counter, "00") & ".pdf"
        Do
            If Dir(CheckName) = "" Then Exit Do   'Filename does not exist
            counter = counter + 1
            CheckName = FilePathAndName & "-Ver" & Format(counter, "00") & ".pdf"
        Loop
    End If
    If counter > 0 Then FilePathAndName = FilePathAndName & "-Ver" & Format(counter, "00")
    
    On Error GoTo FileSaveError
    ActiveSheet.ExportAsFixedFormat Type:=xlTypePDF, Filename:=FilePathAndName
    On Error GoTo 0

    GoTo ExitThisSub

FileSaveError:
    Response = MsgBox("The file path or filename:" & vbCrLf & vbCrLf & FilePathAndName & vbCrLf & vbCrLf & "is INVALID!  Check that:" _
        & vbCrLf & "? Target subdirectory exists" & vbCrLf & "? Filename does not contain invalid characters (such as ?<>/: etc.)" _
        & vbCrLf & "Edit the Reference entry in the source form to remove any invalid characters.", vbOKOnly, "File Save Error")
    
ExitThisSub:
    
End Sub

More info:
The macro crashes on the If Dir(CheckName) <> "" statement (to check if the filename already exists).
At that point, the filepath for CheckName is https://company1.sharepoint.com/sites/Projects/Shared Documents....

Does the Dir command fail when using filepath that is a URL? Is there some workaround?

More info:
I found some info online about converting URLs to UNCs. I added code to the macro so that now the filepath for CheckName is:
\\company1.sharepoint.com\sites\Projects\Shared Documents....
I still get the Error 52 (bad file name) when running the DIR(CheckName) statement.
Any ideas on why this filepath is considered "bad"? Again, there are no prohibited characters in the path or the checked filename.