Guys,
Thanks for your replies. I forgot to mention what I haven't tried yet - which is the bootloader burning.
The reason I'm "contemplating" this step is because I don't know if my bootloader works ok - the L led flashes all the time with irregular intervals 1-2 seconds. I thought it should be either regular intervals or just once, twice. What is the "standard" behaviour of this led? Can I test/probe/verify this bootloader somehow?
I'm reluctant to go on with burning because I haven't found a definitive guide how to do it for Mega 2560 R3 (or basically a confirmation that it helped to anyone having problem similar to mine, or at least to something other than "!!!" in the code) - I don't want to mess with it if I'm not sure this is the problem with the board.
I think that if the board is responding to loopback test, the main chip seems to be working (correct assumption?)
Cheers,
K.