Thanks for the quick response and the info.
I installed version 1.2.3 before the office filled up with people. It is running now for 2 hours without any issue. The boot process was normal.
I also suspected some hardware hassle. Typically I would assume timing or temperature being involved in such a case. I just do not have any explanation why this happens at the same point during the boot process and never during normal operation.
When the office gets empty I will perform more boot tests to see whether the issue is solved with the new FreeBSD or not.
Christian