thank you for your concern. The problem have disappeared for now.
Firstly i experienced the problem in the monitor, then I - in accordance with postman recommandations - tried newman, where I experienced a similar behavior.
After working with the problem for a few days I found the reason for my trouble; in the monitor my first test (a login) failed because of a configuration change in my program, and the following tests somehow ‘lost the environment’.
My following newman tests failed for a more mundane reason; I forgot an ‘-e’ before environment.
So my troubles with this subject have ended …they were mostly my own.