when commiting in magit it used to open a diff window where I could see the changes I'm commiting right now. For some reason this window doesn't open any more on some repos. I must have disabled this behaviour somehow. Unfortunately I don't know how I did this or how to undo this. Can someone help me out here?
I think you have added something like the following to your configuration:
(remove-hook 'server-switch-hook 'magit-commit-diff)
Simply remove it and the diff popup should be shown again.
Note that you can show the diff manually with
C-c C-d (