error receiving AR invoice

Nathan,
when receiving an invoice from AR, after click on "Post", this error happens:
[Wed May 20 10:47:44 2009] [error] [client 127.0.0.1] egw_minimal::__get: There's NO ADOdb object! egw_minimal::__get / egw_db::connect / bo_perp_api::reconnect_transactional / call_user_func(Array) / ExecMethod(perp_api.bo_perp_api.reconnect_transactional)/ bo_perp_payment::post_payment / ui_perp_payment::payment_new / call_user_func(Array) / ExecMethod(perp_ap.ui_perp_payment.payment_new) / etemplate::process_exec / call_user_func(Array) / ExecMethod(etemplate.etemplate.process_exec), referer: http://127.0.0.1/egroupware/index.php?menuaction=perp_ap.ui_perp_payment.payment_new&supplier_id=24
[Wed May 20 10:47:44 2009] [error] [client 127.0.0.1] egw_session::init_crypt() required PHP extension mcrypt not loaded and can not be loaded, sessions get NOT encrypted!, referer: http://127.0.0.1/egroupware/index.php?menuaction=perp_ap.ui_perp_payment.payment_new&supplier_id=24

Can you help me to understand and solve this?