Sorry for vague title; but I don't know what the cause is.
$ ltrace -e 'SSL*' openssl s_client -connect localhost:4433 -dtls1
openssl->SSLv23_client_method(4, 0x7fffb8c3a5e0, 8, 15) = 0x7f01378b4080
openssl->SSL_library_init(0x7fffb8c3c6f0, 0x46f872, 0, 0) = 1
openssl->SSL_load_error_strings(0x7f0137635640, 0x7f01376a2c3a, 0, 0) = 0
openssl->SSL_CTX_new(0x7f01378b4c80, 0, 0, 0x7fffb8c39e10) = 0x1fc9610
openssl->SSL_CTX_ctrl(0x1fc9610, 32, 0, 0) = 4
openssl->SSL_CTX_ctrl(0x1fc9610, 41, 1, 0) = 0
openssl->SSL_CTX_set_verify(0x1fc9610, 0, 0x44ce00, 0) = 0
openssl->SSL_CTX_load_verify_locations(0x1fc9610, 0, 0, 0) = 0
openssl->SSL_new(0x1fc9610, 0, 0, 8) = 0x1fca3f0
CONNECTED(00000003)
openssl->SSL_version(0x1fca3f0, 20, 0, 0) = 0xfeff
openssl->SSL_set_bio(0x1fca3f0, 0x1fcbcd0, 0x1fcbcd0, -1) = 0
openssl->SSL_set_connect_state(0x1fca3f0, 0x1fcbcd0, 0x1fcbcd0, -1) = 0x7f0137689790
openssl->SSL_get_fd(0x1fca3f0, 0x1fcbcd0, 0x1fcbcd0, -1) = 3
openssl->SSL_version(0x1fca3f0, 1, 16, 0) = 0xfeff
openssl->SSL_ctrl(0x1fca3f0, 73, 0, 0x7fffb8c39de0) = 0
openssl->SSL_state(0x1fca3f0, 0x7fffb8c39de0, 0, 0) = 0x5000
openssl->SSL_ctrl(0x1fca3f0, 12, 0, 0) = 0
openssl->SSL_pending(0x1fca3f0, 12, 0xfffffffffffd28d0, 0) = 0
openssl->SSL_get_fd(0x1fca3f0, 12, 0x1fca890, 0) = 3
openssl->SSL_get_fd(0x1fca3f0, 1, 0, 0x7fffb8c39c74) = 3
openssl->SSL_get_fd(0x1fca3f0, 64, 3, 3) = 3
openssl->SSL_get_fd(0x1fca3f0, 64, 3, 3) = 3
openssl->SSL_version(0x1fca3f0, 0x7fffb8c39e30, 0x7fffb8c39eb0, -1) = 0xfeff
openssl->SSL_ctrl(0x1fca3f0, 74, 0, 0) = 0
openssl->SSL_get_fd(0x1fca3f0, 0x7fffb8c39c40, 0, 0) = 3
openssl->SSL_write(0x1fca3f0, 0x1fbd230, 0, 8^C <no return ...>
_cqueues.so->SSL_load_error_strings(2, 0x7fff25916fd8, 0x7fff25916ff0, 0x7fff25916ff0) = 0
_cqueues.so->SSL_library_init(10, 1, 0x7f3712f378d3, 408) = 1
_cqueues.so->SSLv23_method(0, 1, 0, 469) = 0x7f3713410e80
_cqueues.so->SSL_CTX_new(0x7f3713410e80, 1, 0, 469) = 0x6c5d50
_cqueues.so->SSL_new(0x6c5d50, 0, 0x6bbde0, 0x7f3713c62cc0) = 0x6c8390
_cqueues.so->SSL_ctrl(0x6c8390, 33, 2, 0) = 2
_cqueues.so->SSL_ctrl(0x6c8390, 33, 1, 0) = 3
_cqueues.so->SSL_get_ssl_method(0x6c8390, 33, 1, 0) = 0x7f3713410e80
_cqueues.so->SSLv3_server_method(0x6c8390, 33, 1, 0) = 0x7f3713410c80
_cqueues.so->SSL_ctrl(0x6c8390, 55, 0, 0x68ef50) = 1
_cqueues.so->SSL_CTX_free(0x6c5d50, 0x68ef59, 0x7fff, 3929) = 2
_cqueues.so->SSL_set_fd(0x6c8390, 7, 0, 469) = 1
_cqueues.so->SSL_set_connect_state(0x6c8390, 0x6b2b10, 0x6b2b10, 0x7fff25916554) = 0x7f37131db750
_cqueues.so->SSL_do_handshake(0x6c8390, 0x6b2b10, 0x6b2b10, 0x7fff25916554) = 0xffffffff
_cqueues.so->SSL_get_error(0x6c8390, 0xffffffff, 0x6b2ee8, 0) = 2
_cqueues.so->SSL_do_handshake(0x6c8390, 1, 0, 469) = 0xffffffff
_cqueues.so->SSL_get_error(0x6c8390, 0xffffffff, 0x6b2ee8, 0) = 2