OK, so I´ve somewhat managed to get by this problem thanks to reading this thread:http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1208211518
In his reply, Ausboy mentioned unplugging the USB cable for a bit longer time and re-insert it. If done AND directly after this hitting "Upload to I/O boards" I´ve got it to work and not getting any error messages...
Trying to upload a sketch again, same error. Do a "reset" and hit "upload to I/O boards" directly afterwards = success.
Seems that a reset button is very much needed for Arduino Mini.