Maybe redefine \AMC@qaff?

\makeatletter
\renewcommand\AMC@qaff{\alph{AMCquestionaff})}
\makeatother

Thank you a lot for your fast answer,
but inserting the code -- without the typo? ")" -- in the preamble after loading the automultiplechoice package results now in no numbering at all ...

Please post a starting-point minimal source file.

really sorry, now I think I understand your answer:
I think you mean, your code snippet has to be embedded in the source code of the similar solution ment above, I'll try ...

Here is a file with a minimal source code:

I have to add a \AMCsection command every time I insert a new (super-)question, only to get a new leading number for this super-question and all sub-questions -- although I actually don't really need/want any sections. For (sub-)questions I have to reset the counter with \AMCnumero to get the right second number -- every time a new super-question starts.

And that way sub-questions are numerated as 1.1, 1.2 etc. instead of just a) b) c) ...
Activating your suggested code-snippet results in 1.) and 1.) instead of 1.a) and 1.b) or better just a) b) c)

I think I'm missing some important point ...

This works for me. What is your AMC version?

Auto Multiple Choice 1.2.1 (svn:1403) on Debian (package manager synaptic says, that installed version is the newest available under Debian).

Too old for the suggested solution? (I don't mind to install the newest version by hand, but I'm afraid, that there could be pitfalls when installing it without a package manager.)

With AMC 1.2.1, you can use

\makeatletter
\renewcommand\AMC@qaff{\addtocounter{AMCquestionaff}{1}\alph{AMCquestionaff}}
\makeatother

Yes, now it's working. Thank you really, really a lot for your time and your help.

... but on the answer sheet it is still 1.2 instead of 1.a). Do I need to change another macro for the numbering on the answer sheet?