The corresponding server was equipped with a QMH2462 (HP branded QLA2462) running Ubuntu 8.04.02 Server and 2.6.24.53-24-server kernel and it didn't detect all devices it should. Two paths weren't detected. Because I'm not into FibreChannel stuff, further investigation by the SAN vendor showed that it was a login issue with an older but supported FC-Switch, while the paths over a newer switch didn't have this problem.
Unfortunately no solution was available by the SAN Vendor. Then I installed CentOS 5.2 and everything worked like expected, so the physics and configuration should be ok.