SOLVED! pppd says serial line is not 8 bit clean'!
I finally got the ppp link to work. The problem was, the server side
can be told to use 7bit or 8bit by using different characters right
after the call is answered. A RETURN (the \\d\\d bit) in my chat
script switched it to 7bit, which was the problem. So the kernel
message was exactly right, the receiving end wasn't 8bit clean!
Clever, isn't it?
Thanks to all who tried to help. I appreciate it. I wrongly assumed
that it's a chat or pppd problem, sorry. After finishing my thesis, I
would make my contribution to the Linux and Debian community.