Whilst there probably is many answers to the problem “allowing VPN through ASA”, the following will fix problems with regards clients on the inside trying to establish a VPN-connection (L2TP) to remote peer on the outside.
The logs show the following error: “regular translation creation failed for protocol 47 src such and such”

Due to the fact GRE is portless, and encapsulates IP-packets before it is passed on to the transport layer, the ASA struggles a bit getting a NAT going for these connections.
This can be fixed, however, by inspecting PPTP.

Inspection of PPTP is done as follows

Hope this helps.