Help;-( My serial port /dev/ttyUSB0 is suddenly missing


a few minutes ago, arduino has listed the serial port ttyUSB0.
But then suddenly it disappeared in the serial port menu of arduino.
i restarted my machine, plugged in and out a few times and then for a shurt time the serial port was listed again. But now, a few minutes later it is again disappeared and now i can 't get it back.

When i say

chmod a+rw /dev/ttyUSB0

Terminal answers

Cannot open /dev/ttyUSB0: Permission denied

Does anyone has an idea, how i can get the port back?

Edit: Its very strange:
I have now found out that if the port gets lost, i have to plug the Arduino in (at USB-Port) and then type

chmod a+rw /dev/ttyUSB0

Then the port comes back again.
But after uploading a few sketches in arduino, it gets lost again.
And then the game repeats…typing

chmod a+rw /dev/ttyUSB0

and again it works. So do i have to do this the rest of my life;-) ?
Or is there a reason / solution for this behaviour?

Best wishes

Kernel updates usually requires a restart - did you try to restart your computer?

Also what does lsusb command tell you? Is your device listed?