All was well and good on 3.7.4.3 FW on our Draytel 2860N Router. 2 x VPN's (1 ipsec, 1 PPTP). Tried upgrading late last year to 3.7.4.5 FW but router continually rebooted. Had to downgrade to 3.7.4.3.
Noticed 3.7.8 FW released early in Jan 2015 so thought I'd try it. Upgraded and rebooted without issue. All seems to work except PPTP VPN fails to connect (states CHAP failure). Couldnt fix so re0installed 3.7.4.3 and worked again immediately.
Draytek - Please resolve asap.

I've seen the same with regular Lan-LAN IPsec VPN dropouts on 3.7.8

Yes can confirm have the same problem to, only started since f/w 3.7.8, the VPN IPSec lasted 17 days then dropped, only a reboot of the 2860Vn would resolve. Did reload f/w with .rst and restore from backup, now after 7hrs VPN dropped. It seems only dial out (LAN-LAN) is effected, creating a dialing PPTP (LAN-LAN) works fine.

As per previous comment, Draytek please, please, please resolve ASAP!