Task login redirect loop but other apps are fine

All 6.3 apps work well, except flowable-task which keeps redirecting to the login page:
(a) admin username/password are correct;
(b) using nginx into a virtual machine but addresses look fine;
(c) admin belongs to the "Privileges/workflow app" (idm);
(d) flowable-ui-task-app and application.properties seem fine comparing with the other apps;
(e) other applications login/logout without problems;
(f) no ERROR message in Catalina.out and flowable-task.log (log4j);
(g) a fresh h2 database do not change the behaviour.

Many attempts of changing configurations and reinstalls without success.
Is it a Spring token/cookie issue? Any idea how to solve?
Thks