May I thank all those who have helped me locate the source of the problems with this program, in particular Charles Stagg who, maybe inadvertently, pointed me in exactly the correct direction. He also found another bug in the program, which will be corrected in its next release.
The main problem (not appearing to start and/or giving the "stopped working" message) is caused when the selected MSTS root folder is the root folder of a drive. The .cfig file stores the parent folder of the last selected folder - if itself is the drive's root folder, there is no parent with the result that the second item read from the .cfig file is null, thus causing the "index out of range" message. This explains why the first run after an installation is always successful - the .cfig file is empty and this is allowed for.
The second bug found by Charles occurs when a .wav or .ace file is enclosed in double-quotes, but the closing quotes do not immediately follow the file extension. It results in the error "Invalid character in path" being displayed, meaning the opening double-quotes character.
I apologise for all the hassle this has caused
Cheers,
Ged