testing with debug build, 20 users, I get another assert crash:

#0  __pthread_kill (threadid=<optimized out>, signo=6) at ../nptl/sysdeps/unix/sysv/linux/pthread_kill.c:63
#1  0x00000000008fb46d in my_write_core (sig=6) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/mysys/stacktrace.c:433
#2  0x0000000000781a1a in handle_fatal_signal (sig=6) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/signal_handler.cc:247
#3  <signal handler called>
#4  0x00007f10c64723e5 in __GI_raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#5  0x00007f10c6475b4b in __GI_abort () at abort.c:92
#6  0x0000000000a23dcc in lock_rec_queue_validate (block=0x7f10a389c058, rec=0x7f10a536c396 "\217\264^\241#\255\327?\200", index=0x7f1084034768, offsets=0x7f1091490390) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/lock/lock0lock.c:5238
#7  0x0000000000a253e7 in lock_sec_rec_read_check_and_lock (flags=0, block=0x7f10a389c058, rec=0x7f10a536c396 "\217\264^\241#\255\327?\200", index=0x7f1084034768, offsets=0x7f1091490390, mode=LOCK_X, gap_mode=0, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/lock/lock0lock.c:5885
#8  0x0000000000a5d3a8 in row_ins_set_exclusive_rec_lock (type=0, block=0x7f10a389c058, rec=0x7f10a536c396 "\217\264^\241#\255\327?\200", index=0x7f1084034768, offsets=0x7f1091490390, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:1203
#9  0x0000000000a5e3bf in row_ins_scan_sec_index_for_duplicate (index=0x7f1084034768, entry=0x532b0b0, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:1752
#10 0x0000000000a5ed9f in row_ins_index_entry_low (mode=2, index=0x7f1084034768, entry=0x532b0b0, n_ext=0, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:2102
#11 0x0000000000a5f52c in row_ins_index_entry (index=0x7f1084034768, entry=0x532b0b0, n_ext=0, foreign=1, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:2301
#12 0x0000000000a5f8b3 in row_ins_index_entry_step (node=0x52e1ea0, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:2389
#13 0x0000000000a5fb72 in row_ins (node=0x52e1ea0, thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:2521
#14 0x0000000000a5fe77 in row_ins_step (thr=0x52e2150) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0ins.c:2638
#15 0x0000000000961659 in row_insert_for_mysql (mysql_rec=0x5439d40 "\341\035", prebuilt=0x52e16e8) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/row/row0mysql.c:1245
#16 0x000000000093a965 in ha_innobase::write_row (this=0x53deff0, record=0x5439d40 "\341\035") at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/storage/innobase/handler/ha_innodb.cc:5619
#17 0x000000000078bbee in handler::ha_write_row (this=0x53deff0, buf=0x5439d40 "\341\035") at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/handler.cc:5257
#18 0x000000000085107f in Rows_log_event::write_row (this=0x7f10881a8ff0, rli=0x7f108c007f20, overwrite=false) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/log_event.cc:9242
#19 0x00000000008511e8 in Write_rows_log_event::do_exec_row (this=0x7f10881a8ff0, rli=0x7f108c007f20) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/log_event.cc:9413
#20 0x000000000084d6d4 in Rows_log_event::do_apply_event (this=0x7f10881a8ff0, rli=0x7f108c007f20) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/log_event.cc:8054
#21 0x0000000000585f68 in Log_event::apply_event (this=0x7f10881a8ff0, rli=0x7f108c007f20) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/log_event.h:1139
#22 0x000000000060ef9e in wsrep_apply_rbr (thd=0x3957c00, rbr_buf=0x7f108810b7e0 "\006 LQ\023\003", buf_len=306) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/sql_parse.cc:7900
#23 0x000000000060f58c in wsrep_apply_cb (ctx=0x3957c00, buf=0x7f108810b7e0, buf_len=473, global_seqno=42530) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/sql_parse.cc:7979
#24 0x00007f109b7a33bf in apply_wscoll (trx=..., apply_cb=0x60f4de <wsrep_apply_cb(void*, void const*, unsigned long, long)>, recv_ctx=0x3957c00) at galera/src/replicator_smm.cpp:36
#25 apply_trx_ws (recv_ctx=0x3957c00, apply_cb=0x60f4de <wsrep_apply_cb(void*, void const*, unsigned long, long)>, commit_cb=0x60f7e9 <wsrep_commit_cb(void*, long, bool)>, trx=...) at galera/src/replicator_smm.cpp:80
#26 0x00007f109b7a8c70 in galera::ReplicatorSMM::apply_trx (this=<optimized out>, recv_ctx=0x3957c00, trx=0x7f10880a46c0) at galera/src/replicator_smm.cpp:469
#27 0x00007f109b7a9400 in galera::ReplicatorSMM::process_trx (this=0x3933f40, recv_ctx=0x3957c00, trx=0x7f10880a46c0) at galera/src/replicator_smm.cpp:1071
#28 0x00007f109b786304 in galera::GcsActionSource::dispatch (this=0x39344c8, recv_ctx=0x3957c00, act=<optimized out>) at galera/src/gcs_action_source.cpp:110
#29 0x00007f109b786f83 in galera::GcsActionSource::process (this=0x39344c8, recv_ctx=0x3957c00) at galera/src/gcs_action_source.cpp:168
#30 0x00007f109b7aafdd in async_recv (recv_ctx=0x3957c00, this=0x3933f40) at galera/src/replicator_smm.cpp:374
#31 galera::ReplicatorSMM::async_recv (this=0x3933f40, recv_ctx=0x3957c00) at galera/src/replicator_smm.cpp:355
#32 0x00007f109b7bebbe in galera_recv (gh=<optimized out>, recv_ctx=<optimized out>) at galera/src/wsrep_provider.cpp:201
#33 0x000000000060fb03 in wsrep_replication_process (thd=0x3957c00) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/sql_parse.cc:8132
#34 0x000000000055e90a in start_wsrep_THD (arg=0x60fa80) at /home/seppo/work/wsrep/wsrep-5.5-23-new/wsrep-5.5-23/sql/mysqld.cc:4416
#35 0x00007f10c67e6efc in start_thread (arg=0x7f1091495700) at pthread_create.c:304
#36 0x00007f10c6520f8d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112

This is debug level assert, and may relate to the underlying problem.