Mysql – Replication server crash – > 18:39:32 UTC – thesqld got signal 11

Below you can see the part of the log showing the crash. The replication server is 5.6 and the master 5.5 but this setup has worked with warnings for the last week or so.
Now, after trying to change some user permissions, the replication server crashed and won't load again. Any ideas on where to start?

18:39:32 UTC – mysqld got signal 11 ; This could be because you hit a
bug. It is also possible that this binary or one of the libraries it
was linked against is corrupt, improperly built, or misconfigured.
This error can also be caused by malfunctioning hardware. We will try
our best to scrape up some info that will hopefully help diagnose the
problem, but since we have already crashed, something is definitely
wrong and this may fail.

key_buffer_size=262144 read_buffer_size=524288 max_used_connections=1
max_threads=200 thread_count=2 connection_count=0 It is possible that
mysqld could use up to key_buffer_size + (read_buffer_size +
sort_buffer_size)*max_threads = 156591 K bytes of memory Hope that's
ok; if not, decrease some variables in the equation.

Thread pointer: 0x7fb01c000990 Attempting backtrace. You can use the
following information to find out where mysqld died. If you see no
messages after this, something went terribly wrong… stack_bottom =
7fb0387649f0 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x20)[0x83f710]
/usr/sbin/mysqld(handle_fatal_signal+0x34d)[0x61fbbd]
/lib64/libpthread.so.0(+0x11240)[0x7fb2739b9240]
/usr/sbin/mysqld[0x641849]
/usr/sbin/mysqld(_Z17mysql_create_userP3THDR4ListI11st_lex_userE+0x25d)[0x6467bd]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0x201c)[0x68cc1c]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_state+0x328)[0x691858]
/usr/sbin/mysqld(_ZN15Query_log_event14do_apply_eventEPK14Relay_log_infoPKcj+0xc75)[0x7e5e95]
/usr/sbin/mysqld(_ZN9Log_event11apply_eventEP14Relay_log_info+0x6b)[0x7e3ecb]
/usr/sbin/mysqld(_Z26apply_event_and_update_posPP9Log_eventP3THDP14Relay_log_info+0x25c)[0x813cbc]
/usr/sbin/mysqld(handle_slave_sql+0xde9)[0x816dc9]
/usr/sbin/mysqld(pfs_spawn_thread+0x123)[0xa224a3]
/lib64/libpthread.so.0(+0x91da)[0x7fb2739b11da]
/lib64/libc.so.6(clone+0x6d)[0x7fb2730ec9cd]

Trying to get some variables. Some pointers may be invalid and cause
the dump to abort. Query (7fb01c021a8a): is an invalid pointer
Connection ID (thread ID): 2 Status: NOT_KILLED

The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html
contains information that should help you find out what is causing the
crash.