Wrong form displayed after login

[studiomiazzo]

Hi everybodies,
we're facing a strange behaviour we cannot understand completely.
Sometimes the form that is displayed after the login is not the normal initial form but another one from the forms we deployed.

Here are some points :
1. the form displayed is always the same (and we're sure it doesn't depend on alphabetically order)
2. the form displayed doesn't appear in existent solutions as 'firstForm'
3. this often happens when we try to logon with 2 different users (for example we are connected with USER_1, we logoff, we try to login with USER_2 from the same browser on the same machine)
4. we experienced it also with the same user on two different machines

Had anyone got the same problem?
Is there an explication of application server users' logging?

Thank you in advance.