>>In such cases, just adding one line (empty, or a comment) or moving some code up and down was the cure. I have no explanation, and since it happens once a year or less often, I don't really care. First time it took me two hours to find out; second time it was about half an hour. Nowadays, even though it doesn't happen as often (maybe the last time was 18 months ago), I just add a comment and it starts working.
>>
>>My only explanation is that some combinations leave the compiler in an unhapy state... maybe it's a Christian and it gets scared when object code's mod(checksum, 0xfff)=0x666, who knows? Anything that throws the checksum off the doomed value fixes it.
>>
>>And yes, all the electronic chips are powered by magic smoke. Proof? When the smoke goes out, they're dead.
>
>Thanks for the reply. I am glad to know that I am not the only one with such problems. Your explanation sounds good to me. The problem acctually also occured if I would put in empty lines in my code. The only change to main.prg would be an empty line and the dll wouldn't build correctly.

Well, maybe I don't remember correctly about the empty line - I'd rather put a comment line there, stating "the following line was much later, moved it here to circumvent a fox bug" - and adding that line (or moving the other, or both) changed the magic number to a good one :).