IDE 1.6.10 An error occurred while uploading the sketch, 1.6.9 works OK

Had this problem, changing the programmer to AVR ISP worked for me

DrAzzy: 1.6.10 is a shitshow of a release.

It apparent that the IDE developers do not bother with QA, and have a lackadaisical attitude towards even the most severe bugs (why are the unusable releases not pulled from the website, like any company would if they put out a bad build)

I have Arduino Pro Mini with serial to RS485 board, in PC with Windows I have USB to RS485 dongle and I'm using this system to upload code to Arduino for convenience (I need to have connected RS485 to PC anyway). It works fine with 1.6.9, of course I need press reset button when uploading.