osize too small

I'm using ppp with pptp to connect to a MS server at work. Kernel is 2.6.3 AMD64 on Mandrake 10.0, ppp 2.4.2. I built this kernel myself to
fix the 64-bit issues with ppp (fix previously reported.) When I have
VPN up and run a browser, it will retrieve a particular page fine the first time. However, if I click the submit button on that page again (which transmits a bunch of data - I authored this page so know all about it) the browser just goes into continuous wait and never actually transmits anything. I see the following in dmesg:

PPP generic driver version 2.4.2
ppp_mppe: module license 'BSD without advertisement clause' taints kernel.
PPP MPPE Compression module registered
Module ppp_mppe cannot be unloaded due to unsafe usage in include/linux/module.h:503
mppe_compress[0]: osize too small! (have: 1400 need: 1404)
ppp: compressor dropped pkt
mppe_compress[0]: osize too small! (have: 1400 need: 1404)
ppp: compressor dropped pkt

What is the appropriate fix for this? ifconfig shows an MTU of 1396 for ppp0. I tried specifying mtu 1500 when starting pppd, but it still ends up 1396. I suppose the server end is constraining it. Appreciate all suggestions - I can rebuild the kernel if necessary.

Guy Rouillier