Cant login after update

I am on i3
I read somewhere that I should startx then i3-config-wizard
Edit: it was here
after renaming my config to config.bak and then running i3-config-wizard I am back into i3
So it must be something in my (modified) config file I guess
Do you know where that could be to stop i3 starting and getting the behaviour that I had, please?

EDIT:
I realised finally that this was i3 not starting and looked in that part of the forum and found the post about picom

when I commented out that line I am back in

Now to understand what I did what picom is and how to get things working ‘normally again’ which I will do on the i3 part

Thanks for your help!