Thank you for answering so fast.

Mamarok wrote:You need to make sure you do not have special characters in the filename, as those have much tighter restrictions than the ID3 tags where accented characters should cause no problems.

I have checked. Just accent, no other special characters.

When ripping with K3b, you can restrict the filename use to your system settings which avoids special characters in filenames.

I also re-ripped the same CD in the same machine. Same result.

As for UTF-8: of course this only works if your system also is UTF-8 only, you should check your LOCALE settings. See also https://help.ubuntu.com/community/Locale

My locale are set to utf-8.

Yet there is something strange. I have the same directory (~/Music) copied in 3 different machines. The 3 of them the same configuration (Kubuntu, KDE). I synchronized them with rsync. In one machine (this at work) the directory with the album missing is displayed correctly!
I am copying now the result of locale, a screenshot of the locale configured in the System settings, and a copy of the /etc/default/locale. I will also copy the .kde/.../amarok* to compare with the same results/files at the other computers (at home) to compare.
The same directory, with the same Amarok, KDE and Kubuntu version behave different
It must be something encoding related... some setting I will keep on looking, now I have to do some work
thank you for your time, I'll let you know if I can solve it (or not).

raul