Fixing SquirrelMail
From Antiflux Wiki
When <a href="https://mail.antiflux.org/">SquirrelMail</a> complains about:
ERROR: ERROR : Could not complete request. Query: THREAD REFERENCES ISO-8859-1 ALL Reason Given: THREAD failed: No such thread type
the fix is to remove thread_INBOX=1
from /var/lib/squirrelmail/data/«username».pref
.