ISDN connection unstable with Deflate enabled
 

I'm using Slackware current with custom compiled 2.6.18 kernel.
I have an USB ISDN modem and I compiled BSD and Deflate compression into the kernel, but when I connect to my ISP I get disconnected almost instantly when I start transfering data.
KPPP than reconnects with Deflate disabled and everything works fine. If I setup pppd with nodeflate option, everything works great.
My question is why is this happening, because when I had 2.6.17.13 everything worked like a charm with compression enabled?
Also in dmesg I get and error like