Unable to debug sieve

I'm trying to enable sieve debug with no success on dovecot 4.0. I would like to know what it is doing when taking decisions if running a script or not and what it does when a script is executed

These settings do not enable the trace

sieve_trace_debug = yes sieve_trace_dir = /tmp (I also tried any other world writable dir and also ~/.sieve while testing trace for personal scripts)

nor this one is enabling the debug

log_debug = category=sieve

sieve is working perfectly for personal but not for global scripts.

Any hint appreciated. Thanks in advance, Roberto