Thanks for the feedback.

Unfortunately, it doesn't quite do it. When trying the filter, httrack finds
all sorts of unwanted files with this threadnumber. My impression is that
httrack only looks in the current library, which is packed with all the
messages of the complete forum, but does not follow the links on the startpage
of the scan. (I may be wrong on this.) Thus, it doesn't seem to resolve the
mapping "next message" -> message id on server.

In order to screen out the unwanted files, I edited the proposed filter
slightly:

-* +*=18363* -*10* -*userid*  

the result is that most unwanted files are skipped, but now httrack just saves
the start page and then ends the session. When looking at the mirrored page,
the "next" link is translated to the next message, but this next message link
points to the original site.

Any ideas to solve this?
/Sven