Hi Raoul,
at first i would enable a BR*Tools trace (BR_TRACE 15) to cross-check the DBMS_STATS call by brconnect. Afterwards i would check DBMS_STATS by tracing it, if everything was alright with BR*Tools (parameter degree is correctly set). Please be aware that DBMS_STATS may use serial execution regardless what you specified for parameter "degree".
http://docs.oracle.com/cd/E11882_01/appdev.112/e40758/d_stats.htm#ARPLS68582
degree
When using DEGREE=>NULL, DEGREE=>n, or DEGREE=>DBMS_STATS.DEFAULT_DEGREE, the current implementation of DBMS_STATS may use serial execution if the size of the object does not warrant parallel execution.
This may fit to your statement "doing some tests on our QAS database (much smaller size), trying to update in parallel the statistics of table GLPCA, through several threads, but it does not work". However just trace it and you gonna see the root cause 
Regards
Stefan
P.S.: By the way setting event "38028" disables small object optimization for dbms_stats.