I had a dual monitor configuration going on for quite some time. One day, after moving the computer to another location and reconnecting everything, it changed such that I saw everything in double (being very much sober), I think it's called the 'mirrors' config. Anyway, from there on, there was nothing to be done through the system settings gui to change it back, as it wouldn't allow me to save any modification. The error I get when clicking 'save' is : "Failed to create file /home/me/monitors.xml.xxxxxx. Permission denied", xxxx being a random code, changing everytime. However, I can save all the configurations I want just fine by using the terminal, in my case:

 xrandr --output DVI-I-1 --right-of VGA-1

So I do have a workaround and this is therefore a question more out of curiosity. What could possibly have changed to make it impossible to do it through the gui and still letting me change the config using xrandr without being root? I'm having a hard time believing it could have anything to do with disconnecting/reconnecting the monitors... Any idea? Thanks !