Simple as reportbug
Unless you know which package to report the bug against, you can report the bug against the xorg metapackage:
Like most packages related to the X server, reporting a bug against this package triggers a bug script which is going to collect X-related information, be it installed packages, running kernel, X configuration, X log, and so on.
If a backtrace shows up in the X log, it’s much appreciated to try and get a full backtrace using gdb.
Follow-up with more info
If you reported a bug against another package and if that bug was reassigned to an X-related package, we might need more information. You can run the bug script manually and attach its output to your mail to the bug report: