I have built the following arrangement: RB750 (V4.2 PPTP Server + L2TP Server) connected to several RB4XX (V4.2 PPTP Client + L2TP Client).

Every client creates a PPTP and a L2TP tunnel to the server, through the same link and with equivalent settings.

L2TP tunnels stay connected forewer. PPTP tunnels die after a few days, one client after the other. Some clients never die.

"died" PPTP clients still try to connect to the server, but they get discconected during authentication process.

Downgrading the server to 3.30 and upgrading to 4.2 brings PPTP up again. Rebooting only the server doesnt produce any change. Really weird.