Thanks for your reply!
The answers are: yes -- no.
It is as follows:
Mouse, keyboard and printer always work when connected to the port.
When the USB hub fails, all devices fail when they are connected to the /hub/
(mouse, keyboard, printer and flash drive). In this case, keyboard, printer
and mouse work while the flash drive fails when connected to the /port/.
When the USB hub works, all devices, including the flash drives, are working
directly on the /ports/ and on the /hub/.
I never had any problems connecting USB flash drives running Ubuntu. If I
remember correctly, the problem started with flash drives not being
recognized when connected to the /port/ after I had changed to Trisquel a
couple of months ago.
My system is an IBM Thinkpad T40 with a 1.5GHz pentium CPU; I'm not sure when
it was build. I have the feeling it's a quite reliable machine. Do you have
an idea?