Thank you both!!!
I guess I somehow managed to solve the problem by:
(i) completely removing packages from Autounattend.xml
(ii) dropping Autounattend.xml onto a separate USB stick.
After that unattended setup completed successfully! Dunno why but whatever I do, setup refuses to read Autounattend.xml directly from the root of installation DVD. Actually, it reads it and follows instructions partially (judging from "skipped" windows during setup) but pops up with the above message at some point after expansion of Windows files. Anyway, I'll stick to the above scheme for now, and will share future problems/achievements with you here...
Now, let me ask a completely different question which occurred during the last successful installation... I cannot implement different keyboard input languages. As you can see from Autounattend.xml (see: my previous post above), the XML contains the following:
<component name="Microsoft-Windows-International-Core" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<InputLocale>0409:00000409; 042c:0000042c; 0419:00000419</InputLocale>
But in fact, on first logon to Desktop I cannot see English, Azeri & Russian set as keyboard input locales on the language bar. Checked it on Regional settings page - the same thing, only en-US is defined. System locale (Russian) is OK though... Input locales - no changes!
Any advice?!? Maybe I need to apply these settings at different pass? Thank you!