GA-8IEXP F4 BIOS locks with UPS serial cable connected

I bought a Mustek PowerMust 1000 online UPS and connected my pc to it.

If I connect DB9 serial cable provided with the UPS, BIOS freezes.
It displays BIOS revision on screen but hangs before beginning memory
test, then I can only hardware-reset my pc or turn it off.
It's impossible to enter BIOS setup too.

If I disconnect serial cable, BIOS boots correctly.

As you all perfectly know, serial connection is vital for monitoring
UPS activity and use its features to turn off pc in case of power
failure.

I checked serial cable pins with a tester and noticed they all nine are
connected: it seems a one-to-one connection wire (pin 1 male connects
to pin 1 female, pin 2 male connects to pin 2 female, and so on).

I contacted Mustek support and they told me it's a motherboard problem.
They also suggested me to configure BIOS to restart pc when power comes
back from a failure. I did so but was unsuccessful.

Finally I tried to connect UPS serial cable to another pc without
setting anything and it works greatly.

So, why not with my pc? Do you have ideas, suggestions?

Thanks to all.