I've filed http://issues.roundup-tracker.org/issue2550575 which stefan on
#roundup  oftc commented was probably a known bug.

I would just add this as a comment to issue1526 but we can't view that in the
web interface anymore (due to my accidentally pasting in the message of doom)

This looks fairly simple to explain.  

The bit of code that hyperlinks things that look like objects (eg. "issue 1526")
got confused by "file SOMEHUGENUMBER", where SOMEHUGENUMBER was the file name
corresponding to the object.

Workaround : use roundup-admin to remove the offending message, eg:
> display issue1526
> set issue1526 messages=-8175
> commit

(which I have done for issue1526 and will do for issue220).

Once I fix issue220 with this workaround, I will consider the present issue
resolved (because it's not really a persistent problem, maybe an occasional
nuisance in the future, but just a one-off thing)