Dear Rick,
thank you for taking your time again in order to help me here. I just tried your slightly changed code and it worked via Zsh! No errors or problems occured. And I didn't know that square brackets are treated just like wildcards by AFNI. I knew about the * and ? wildcard option. But, as I said, I did not know that the same applies for the square brackets. And this is why I have to add them again for the input filename, i.e., at the end of the code for 3dTcat.
Thank you and see you around the board,
Philipp