You should not have Menu>Load>Sysex !! This is now useless. The preenfm2 is always ready to receive success.
What firmware do you have ?
Seems like you flashed an old one.
Well, you know... Since i thought i had to flash the bootloader
& firmware the DFU way, i flashed the 0.9 version packed with the bootloader. Don't ask me why, that's the "Jumper 3 trap"
Anyway : firmware upgraded from USB (found the file transfer speed was good), DX7 banks loaded, everything's fine, sounds great. Now i'm up to link it with my BCR controler and try some NRPN learning
Thanks again Xavier, best consumer service ever