Hello s.dinah,
The problem is with the API call CreateFile. This call is limited by the string convention used for identifying the COM port. If the COM names used are like COM1, COM2, etc. the highest COM port is COM9. Not sure why this is but I suspect it is a throwback to the early days of computing.
Anyway, the good news is you can use a simple workaround by prefixing the COM port string with "\\.\" and the COM name like "\\.\COM1", "\\.\COM2", "\\.\COM10".
The line of code you need to change is in the function CommOpen located in modCOMM module.
Here is the updated line of code...
' Open serial port.
udtPorts(intPortID).lngHandle = CreateFile("\\.\" & strPort, GENERIC_READ Or _
GENERIC_WRITE, 0, ByVal 0&, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0)