Thanks for you reply Jimbuna and Danlisa, but I'm afraid it won't solve my problem. I'm pretty sure that when I received a convoy contact report via radio the TC didn't change a bit (despite it was definitely set to drop back to 8x when receiving radio report). I played the game on 256x TC and when I noticed the new big square on the navmap it was hours after the radio contact report...so it is pretty sure that the TC remained at 256x despite the radio contact report receival. (The only thing that MIGHT have happened is that I didn't notice that a new radio contact report was received and set the TC level to 256x and only after a while noticed the map contact update. But this is quite unlikely, I think. )
And it is also interesting that when I receive not-so-important radio messages (messages that were added by the GWX team to improve immersion e.g. Radio Berlin broadcasts, S.O.S. signals from distant ships, other Uboats' status reports, etc.) the TC always drops back to 1x (!) and I hear the Radioman announcing the reception of a new radio message. Is this type of radio message dealt in a different manner than the radio contact reports (and "Be more aggressive!" and "Keep up the good work" type of messages)? (That would explicate that the TC drops back to 1x at these messages despite it should drop back to 8x.)
Sorry for the too much philosophy.
Anyway I set the TC at radio report receival to 1x in SH3 and give it another try. Who knows, it might work this time.
Matyas