There appear to be routers out there (Archer C9 for example) which support PPPoE and not mppe, but for some daft reason they send an mppe request with all options turned off, i.e. they request mppe with all options off, and pppd says - rejected, even though that's actually what it's already doing.

The connection works, but with debug on you get a continual stream of

 rcvd [CCP ConfReq id=0x1 <mppe -H -M -S -L -D -C>]
 sent [CCP ConfRej id=0x1 <mppe -H -M -S -L -D -C>]

messages every few seconds, which seems wasteful if nothing else. It's easy enough to fix this in the code by just accepting these mppe requests even if mppe is disabled, and I wondered if there was any interest in fixing this - I'd be happy to contribute a patch if so.