Thanks to Stefano Zacchiroli who tested and installed COCANWIKI with PostgreSQL 8 (in his case, 8.1). This revealed a number of issues with the default installation method.
The SQL dump file provided with COCANWIKI actually contains tables which are created as part of tsearch2, but the version which I have installed (ie. 7.4). This version uses OIDs, but later versions do not.
One solution to this is to modify the CREATE TABLE commands to add WITH OIDS to each.
Another is to strip out the SQL dump file of any tsearch2-related code and install that separately with a more recent tsearch2.
Eventually I will try to do this in the official distribution (if you want to help out, let me know).
This is another tsearch2-related problem.
If you are using a non-US-English locale, then you need to populate pg_ts_cfg. This should help you out:
http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/docs/tsearch2_german_utf8.html