Access is denied only sometimes

I can't figure out how this can work.

I own a Genuino Uno, and sometimes I can upload my sketch to it, but some other times (most of the times unfortunately) it just gives the famous error:

avrdude: ser_open(): can't open device "\.\COM1": Access is denied.

And I really can't understand the logic in that
Why does it only work rarely?

Which version of the Arduino IDE are you using?

pert:
Which version of the Arduino IDE are you using?

1.6.8

Are you using Windows? I think it's the same problem as: Arduino 1.6.8 constantly polling serial, resetting Arduino board, Access Denied - Installation & Troubleshooting - Arduino Forum. This issue is isolated to Arduino IDE 1.6.8 and the Arduino developers are working on a fix for it. So if you want to test out the fixed version of the IDE you can download it from Rework serial ports handling by facchinm · Pull Request #4792 · arduino/Arduino · GitHub(and please post your results) or you can also switch to using Arduino IDE 1.6.7 and that should also fix the problem.

I am using Windows. Will try and edit with the results.

Thanks a lot for your kind help

I downloaded the fixed version of the IDE and it does the exact same thing
Worked the first time, now it doesn't work

Update: Works perfectly with 1.6.7

Thanks a lot!