UPDATE:
I tried to update Xwiki to the last super stable version, but the problem
persists. It loads the new version and tries to display the Distribution
Wizard to start the upgrade proccess, but then the gray background shows up
and there is nothing I can do.
One thing to note is that some users can see their spaces, but the Admin
and the others can not. The guest visitor also can see the register and
logon page. So, I suspect that is something related to just one space.
Another thing I've noticed is that I've deleted the xwiki folder to start
the update proccess as recommended in the documentation. So, It's not a
extension problem or a file problem, I think. It seems to be something with
the Database and javascript.
Should I start a bug report on Jira?
Thank you very much.