paparazzo wrote:But as i switched the site before in diagnostics mode and panel is still not available: How can i turn off manually diag mode ?
Sorry, but I don't quite understand the question. "Diagnostics mode" has nothing to do with panel. If you have diagnostics enabled, that would be inherited from the where you moved it from. If you want to change it (on or off), it should be done from the panel, so you need to get that working first. You can edit custom config options by editing config.user.json file, but this is not intuitive, and you can only remove/exit existing custom values anyway.
paparazzo wrote:And of course, what can i try to get the panel back ? (I just get a blank site or an HTTP 500 error).
Why isn't it working? I can't guess these things. Something with your redirect? Why not try to remove the redirect in config first? It's only useful if you want to redirect to HTTPS anyway. For me to diagnose further, I will need link to website for starters.