I have included some of more environments and commands that I often use in the math.xml file. To keep track of what I have messed up, I wanted to enter a comment before the lines I added. It worked fine for the first two comments. After entering the third comment, when I tried to restart TXC, it fails to start under Windows XP Prof with the error message:
"unknown software exception" (0x80000003) appeared in the application at 0x004641e6
Deleting the comments or moving them to the end of the file helped! Now I tried to enter just one comment in the middle of the .xml-file and it crashes again. So there seems to be a problem with comments that are in the middle of the math.xml-file.
It also might be that I just don't know anything about xml...