Hello @tgtje thanks for your support!
Well, after your repply I've continued doing more testing, and really this is very rare.
If I enter to the forum without logging, it works without problems. I can enter to any subforum, open threads, etc.
If I log-in to any account (using the users panel of the administration and using the button of loggin-as-selected-user) the forum works too!
But, If I sign-in with my personal account, that have admin rights, the forum issue appear again... What is happen here?
And well. With other testing stuf, I deleted all the cache content and set it to off, and try to disable the developer mode... result? The sign-in form of the main site disapear, and if I try to log-in on the admin page, or forcing to enter to the regular sign-in page, after enter my credentials says that doesn't match with any registered user... what? The row in the database of my user can be corrupted?
Sorry for the long post; if you know something, please tell me to try that. If i can't found anythin, well... restore database backup to continue playing with it.
Again, thanks for your valued support.