I had a look at the log file of the other side. It's a NetBSD based
firewall with isakmpd.
The log says:
Oct 21 19:57:01 isakmpd: message_recv: invalid cookie(s)
Oct 21 19:57:01 isakmpd: dropped message from 188.8.131.52 port
500 due to notification type INVALID_COOKIE
I will try and play with the old SA setting in m0n0wall...