Error 1064: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'varchar(255) NOT NULL default '' AFTER `timestamp`' at line 1
Topic on Extension talk:WhosOnline
This appears to have been fixed in the 1.40 release, but for anyone facing this issue on the 1.39 release, you have to replace the line in "sql/patch-add-wikiid-field" with:
ALTER TABLE /*_*/online ADD COLUMN wikiid varchar(255) NOT NULL default '' AFTER `timestamp`;