PPPoE troubleshooting

I’ve got problems with RB as PPPoE server: clients suddenly disconnect.
MTU/MRU is 1480, but only some clients are able to connect. I’ve following messages from MK

jul/18/2006 12:24:55 pppoe,ppp,debug,packet <0042>: sent LCP EchoReq id=0x5
jul/18/2006 12:24:55 pppoe,ppp,debug,packet <magic 0x15b0a088>
jul/18/2006 12:24:55 pppoe,ppp,debug,packet <0042>: rcvd LCP EchoRep id=0x5
jul/18/2006 12:24:55 pppoe,ppp,debug,packet <magic 0xec36c463>
jul/18/2006 12:25:01 pppoe,ppp,debug,packet <0034>: sent LCP EchoReq id=0xf8
jul/18/2006 12:25:01 pppoe,ppp,debug,packet <magic 0x5036a417>
jul/18/2006 12:25:01 pppoe,ppp,debug,packet <0034>: rcvd LCP EchoRep id=0xf8
jul/18/2006 12:25:01 pppoe,ppp,debug,packet <magic 0xa04c7856>
Terminal ansi detected, using single line input mode
echo: pppoe,ppp,debug,packet <0034>: rcvd LCP EchoReq id=0xcf
echo: pppoe,ppp,debug,packet <magic 0xa04c7856>
echo: pppoe,ppp,debug,packet <0034>: sent LCP EchoRep id=0xcf
echo: pppoe,ppp,debug,packet <magic 0x5036a417>

Does anybody have an idea what the problem could be?
Thank you