[ubuntu] win8.1 guest won't open with com1 enabled

I have an ubuntu (linux mint 16 64bit) host with virtualbox running xp and it starts up fine with an attached com1 port, which I need. Today I purchased windows 8.1 64 bit as shortly I won't be able to get xp updates. When I set the 8.1 guest to use the com1 port (with as far as I can tell) the same settings as in the xp guest, the 8.1 guest will not open. If I disable the com1 in settings, the 8.1 guest will open fine. Tried a number of times with the same result. The error message on non-startup is, Failed to open a session for the virtual machine windows8.1.
Failed to open host device '/dev/ttys0' (VERR_FILE_NOT_FOUND). And the "details" are,
Result Code:
NS_ERROR_FAILURE (0x80004005)
Component:
Console
Interface:
IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}

Below are screenshots from vboxmanage and from the Settings page:

Attached Images

• com1.png (32.7 KB)
• uart.png (14.0 KB)