Hmmmh... how embarrassing. Looks like I got it solved.
First of all, replacing "-pas" by "-pass" did not solve the problem.

Today I got quite distressed because not any order of the options,
with our without the -y switch, did work. I always received the error
message.

To make a long story short, it seems that it somehow has to do with
how I copied the command into the win-7 console (cmd).

I found that copying the command into the console was leading to the
problems, while completely typing it myself did not. I went as far as
analysing the copied command sequence for hidden special characters or
weird transcriptions of blanks; nada.

I give up.
For now, I will just type each restore command sequence into the
console. ASAP I will try to create something pre-prepared using Jiri's
provider

>> -pas). This error is occurring because of confusion with -pa[ge_size].

> I will try this ASAP. However, the help on gbak.exe should then be
> updated. It reads "-PAS(SWORD) Firebird password. (gbak.exe -? on
> 2.5.2 32-Bit).

have fun,
André