Unbelievable!
Im not sure yet, but I think I solved this issue.
The ACL of the database had an Anonymous entry with Author rights to it. This entry is necessary in order to a local database can be tested in a navigator as it would be accessed on web.
I was testing the database on local access and the error did not occur, but when I copyed it to the server that error occurred.
I removed this entry and the error did not appears anymore! The Xpage works on client and web now perfectly.