Mega 2560 upload problem (Blink upload works!)

I want to upload my sketch

Binary sketch size: 8464 bytes (of a 258048 byte maximum)

but always get the error

avrdude: stk500_2_ReceiveMessage(): timeout

it doesn't even state "Problem uploading to board" or something, it stays at "Uploading to I/O board" forever...

if I quit Arduino software then and want to upload the Blink sketch, I have to manually kill the avrdude process first, if I don't I get

avrdude: stk500_2_ReceiveMessage(): timeout

as well...

please help, I don't know what I'm doing wrong (nothing is connected to the board, I think I tried everything)

thanks in advance buggybug

Does nobody know what the issue could be???

I also sometimes get the error

avrdude: stk500_2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer