In 1.4.x branch SquirrelMail Sent Subfolders plugin creates year folder without delimiter between $sent_subfolders_base and $year. It creates invalid folders when quarterly or monthly option is selected.
Attached patch also fixes marking of special folders.
For example of current marking:
INBOX.Sent - marked as special (correct)
INBOX.Sent.2006.12 - marked as special (correct)
INBOX.Sentit - marked as special (not correct)