Not sure what happened and no idea where to start with troubleshooting. I could say that this is happening after I did the last update, but again, no idea how it could have been happening.
Usually, I leave my PC on at all times but for the last 2-3 days, if I don’t touch the PC for a few hours, when I come back I find the xfce4-display-settings app open in multiple instances, 3-4-5 instances. This is confusing!
Go into /usr/bin and rename xfce4-display-settings and see if there is an error message next time it’s called.
shot in the dark.
8bit
It’s done, now I’ll just have to wait. 
In the advanced settings, turn off the options to show the dialog when a new display is connected ( Xfce4 display settings
External monitor? Check the cabling.
Didn’t you recently move to a remote island paradise? How’s the power grid there? Maybe your monitor is seeing some power fluctuations and the PC sees it as connect/disconnect.
I renamed the executable and in the last 20 hours no more problems. Yes I’m on a PC with an external monitor, I checked the cables again now, and the power is fine because I’m using a ups. I just disabled the dialog for now. One thing I noticed is that it didn’t keep my refresh rate and I had to change it manually again, we’ll see if its happening again…
