Apache2 "Response header name '<!--' contains invalid characters, aborting request"

The answers from @eorochena and @dogacan are special cases. In general:

You get this error if an exception is raised in a Python CGI script.

A good way of figuring out what went wrong is to invoke Python's CGI module debug helper function at the beginning of your CGI script like this:

cgitb.enable(display=0, logdir=OUTDIR)

where OUTDIR is a directory name. If your CGI scripts raises some exception, then Apache puts an HTML file into that directory. The file has some garbage name such as tmpw4olz3xr.html, and at its end it contains the Python stack trace enclosed within HTML comments (<!-- ... -->). This is the information that will help you fix the problem.

Notes:

1. The display=0 parameter means that the error details are not shown in the browser to your users.
2. You should probably comment out cgitb.enable(...) when you are sure your script works OK.