I started getting this error on sketch upload - then tried another PC and it worked. Then tried the original PC and it worked. Now trying both, and getting the error again on both. Different cable too.
Sometimes it shows some pages loaded. Is my board dud?
I have 1) the same problem (some times); in addition to this, 2) the COM7 sometimes fails and, finally, 3) I am unable to change the baud rate of the communication (Serial monitor).
In the first two I DO manage by using the "switch OFF switch ON" manoeuver (not exactly the computer but disconnecting the cable, then unabling the COM7 port -or mixing these two in underminated proportions and sequence-: normally it all recovers and I keep on working).
(By the way, I've posted several times asking for help to change the baud rate -see image attached-. ¿Would anybody help me?).
In my -humble- opnion what happens is that, as Valentín Canadell stated 25 years ago, "computing is not an exact science" (or, better, he said "I thougth computing was an exact science" just after noticing it is not
(Just for spanish speakers: Valentín Canadell dijo "hostias, yo pensaba que la informática era una ciencia exacta".)