[Serusers] 500 server errror (1/SL) from ser while INVITE

Hi, I am trying to build SER 0.8.14 in my office everything is ok, but after 2 weeks more, SER became strange all clients can register to SER, but when INVITE, SER will reply 500 I'm terribly sorry, server error occured (1/SL) and in syslog, Oct 12 16:00:32 localhost daemon.err /sbin/ser[1073]: ERROR: sip_msg_cloner: cannot allocate memory Oct 12 16:00:32 localhost daemon.err /sbin/ser[1073]: ERROR: new_t: out of mem: Oct 12 16:00:32 localhost daemon.err /sbin/ser[1073]: ERROR: t_newtran: new_t failed Oct 12 16:00:32 localhost daemon.err /sbin/ser[1073]: ERROR: sl_reply_error used: I'm terribly sorry, server error occured (1/SL) Is some function in SER need locate large memory , and cause the lack of memory ? but i check the system memory via command free, it still has about 5 mb... have any ideas??? Thanks and regards Jimmy