strange problem: replying to emails stopped working

[stib]

For some reason I'm no longer able to reply to emails. RC was working fine until a few days ago, now I can't send replies. However if I tick the "return Receipt" box it works.

I was using beta build2401, and since then I updated to beta 0.3 (I'm not sure what the build is), but the problem is still there (I installed into a new directory and created new MySQL databases for the new install). My outgoing mail is set to use PHP sendmail(), rather than a specific smtp server. I've got error logging on, but there aren't any errors showing up, just the red notification area alert saying sending message failed when I hit the send button.

Any ideas of what might be going wrong? TIA

[stib]

Oh, if I change the identity when replying I get these errors in my error log

[13-Jun-2009 09:41:54] MDB2 Error: unknown error (-1): _doQuery: [Error message: Could not execute statement]
[Last executed query: PREPARE mdb2_statement_mysql_3588e0431a622d005b0edb5078c31a25433d129a9 FROM 'SELECT * FROM identities\n       WHERE  del<>1\n       AND    user_id=?\n       AND identity_id=3\n       ORDER BY `standard` DESC, name ASC, identity_id ASC']
[Native code: 2006]
[Native message: MySQL server has gone away]

[13-Jun-2009 09:41:54 -0500] DB Error: _doQuery: [Error message: Could not execute statement]
[Last executed query: PREPARE mdb2_statement_mysql_3588e0431a622d005b0edb5078c31a25433d129a9 FROM 'SELECT * FROM identities\n       WHERE  del<>1\n       AND    user_id=?\n       AND identity_id=3\n       ORDER BY `standard` DESC, name ASC, identity_id ASC']
[Native code: 2006]
[Native message: MySQL server has gone away]
 in /home/username/public_html/roundcubemail-0.3-beta/program/include/rcube_mdb2.php on line 269 (POST /roundcubemail-0.3-beta/?_task=mail&_action=send)
[13-Jun-2009 09:41:54] MDB2 Error: unknown error (-1): _doQuery: [Error message: Could not execute statement]
[Last executed query: PREPARE mdb2_statement_mysql_4ad4fe19b1e8b0d14c165a64592de3fcbe0483b84 FROM 'SELECT vars, ip, UNIX_TIMESTAMP(changed) AS changed\n     FROM session\n     WHERE  sess_id=?']
[Native code: 2006]
[Native message: MySQL server has gone away]

[13-Jun-2009 09:41:54 -0500] DB Error: _doQuery: [Error message: Could not execute statement]
[Last executed query: PREPARE mdb2_statement_mysql_4ad4fe19b1e8b0d14c165a64592de3fcbe0483b84 FROM 'SELECT vars, ip, UNIX_TIMESTAMP(changed) AS changed\n     FROM session\n     WHERE  sess_id=?']
[Native code: 2006]
[Native message: MySQL server has gone away]
 in /home/username/public_html/roundcubemail-0.3-beta/program/include/rcube_mdb2.php on line 269 (POST /roundcubemail-0.3-beta/?_task=mail&_action=send)