Hi @ipazin,
thank you, that helped me!
Unfortunately, it causes trouble at later parts of the workflow - output to Excel.
The workflow I have at the moment is:
The output of the grouped list (middle excel sheet) now contains only the Group column (defined in the GroupBy Node).
Approaches to change the list format to string like "Split Collection Column” - “Column combiner” as described here:
do not work for me since I have many columns and I do not want to have additional columns.
Also, I want the data aggregated like in the list format, so that I can reverse the workflow (also when data are exported, altered and imported back to knime) but now I need a way to export the data. 
So in summary; “concatanate” (GroupBy) works for me to reduce the list, but than reversing the workflow does not work and “List” (GroupBy) allows me to reverse the workflow but than I can not export the aggregated list.
Any ideas?
Kind regards and thanks a lot!
Kiro