When an user try to conect to the CB by means the login module of CB, it sends a white page with a message: "Your session has expired. Refresh page and active cookies before filling the form again" (my CB is in Spanish, so this is more or lest the message in English).

Could be this happening because I was trying to install the forum and blog plugings? Would I need to uninstall entirely the CB and reinstall it? Could I lost the whole database users and their details if I unistall CB?

Could somebody help me, please?

First thing is how are you handling sessions, are you using a cache like memcache?
Second, have you tried emptying the sessions table, maybe it's become corrupted?
Third, this is probably a long shot, but have you tried running the tools in the cb admin panel to sync users and check the database tables?