I was able to get a backtrace by running in gdb.

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x43007940 (LWP 30215)]
0x000000367ae796d0 in strlen () from /lib64/libc.so.6
(gdb) bt
#0  0x000000367ae796d0 in strlen () from /lib64/libc.so.6
#1  0x000000367ae46b69 in vfprintf () from /lib64/libc.so.6
#2  0x000000367ae6988a in vsnprintf () from /lib64/libc.so.6
#3  0x0000000000417618 in slon_log (level=<value optimized out>, 
fmt=0x41edd8 " ssy_action_list value: %s\n") at misc.c:175
#4  0x00000000004093d0 in sync_event (node=0x1006d3b0, local_conn=<value 
optimized out>, wd=0x1006cbd0, event=0x1009bfa0) at remote_worker.c:4353
#5  0x000000000040ba42 in remoteWorkerThread_main (cdata=<value 
optimized out>) at remote_worker.c:630
#6  0x000000367ba064a7 in start_thread () from /lib64/libpthread.so.0
#7  0x000000367aed3c2d in clone () from /lib64/libc.so.6




I ran slon with -d 1 and now slon is working again.