I think this problem exist since early version 6 .
I have tested on numerous PPTP VPN server and all of those have the exact same behavior. After say around 20 mins to 30 mins connection, the PPTP debug logging will show this:
It will keep showing the error message in debug "LCP received ProtRej for unsupported protocol"
I usually use the VPN connection for seeing video. When it happens, the video will freeze and the Winbox Interface window will show no activity on the VPN. It seems like the connection freeze or out of sync.
I can consistently reproduce this by connecting to *any* paid VPN service that I subscribe to using the PPTP vpn client and watch any video site that the movie last longer than 30 mins. (eg. youtube), it will eventually throw this error message within 20 to 30 mins.