error 500 after update

[Primo]

because i can't get some help with this i have to reinstall the server again from nothing.
maybe i can get some answers on this:

- what do i have to backup and where? thinking that im going to do a fully new install if debian. like an update? and then copy the backupfiles to another server?

-when i do a backup is there a change that i copy the config again to what is wrong now?

some help would be great...