I solved this problem installing driver for newest USB controller after the error in question appeared.
Here is a theory of the problem. Without appropriate USB driver win7 does not see any flash disks and cannot work properly with USB DVD-drive. Error message says about this fact. So there is the question: how to install USB driver if system cannot work with USB devices and laptop does not have normal SATA DVD drive?
Firstly, I tried to use DVD disk with drivers downloaded from Fujitsu site. This was unsuccessful because win7 installer cannot search correct driver in catalog tree. Exact directory with driver must be provided. But for DVD with drivers it was impossible because installer searches driver in the root only.
I decided to copy Fujitsu drivers to existing win8.1 partition because win7 installer can use it.
1. So, I started win8.1 and copied win7 drivers from Fujitsu site to win8.1 partition.
2. Boot from win7 DVD (BIOS parameters were set according to Fujitsu recommendation).
3. After appearance of error in question I used win8.1 partition to find necessary driver. Directory must be provided exactly because autosearch of drivers does not work. I tried many directories without success but suddenly win7 installed correct driver and installation continued!
4. During win7 installation I completely remove all win8.1 partitions and reformatted SSD disk (win7 did not want to install to GPT disk). I was afraid of missing of drivers and crash of installation process, but win7 installed without additional problems.
I suspect technical support does not provide such instruction to prohibit (with M$ corp) downgrading to windows 7.
P.S. On some systems works fine installation started from DVD plugged in to USB2 port. In the case of my U554 that trick did not work.