If nothing at all is happening, turn the M1 off and then back on. The Midi Ox forum suggests trying it with the buffer size set to 4096, but it still may not work.
As we have previously established, the interface you have uses the built in Microsoft drivers which have are buggy when working with SysEx. However I am using an m-audio MIDIsport interface which uses custom drivers. I have the buffers on my computer set to 512, and the delay set to 120 and have not had any issues. You can also raise the delay number if things still get corrupt.
I'm talking about the "size" of the buffers. In the configure window, I increase the Delay Between Buffers setting to 120 and set the Low Level Output Buffer Size to 128 I make the following settings to slow down transmission so that older keyboards like the M1 don't get overwhelmed with data: - In the Sysex menu of the SysEx View screen I select Configure.
Is this the same method you are using? What is the name of the file you are trying to send? At what point do you get the error? Just a note about settings - this is not the cause of your problem but may matter once you get thing working. Then in the Sysex menu of the SysEx View screen I select File > Send Sysex File. from the View menu of the main MIDI-OX screen. What steps are you going through in MIDI-OX that produce that error message? The way I load the M1 is to select SysEx. As long as nothing is running you are fine. You do not need to uninstall any software.