Could someone please explain me the difference between:
Configuration -> General -> Tab/Indent ->
Tab columns and Indent Columns
F.e. if I set Indent Columns to 2 (Tab Columns is 8) then Increase Line Indent always inserts a tab (8 characters) and not 2 characters.
(What I want to do ist to increase/decrease the line indent by only 2 characters without inserting tabs.)
I am not sure exactly what you want to do. Tab is related to the TAB key on the keyboard, and Indent is usually related to automatic displacement of the cursor when Auto Indent is enabled.