It' s quite impossible to say what' s the matter with that single line.
Negotiation could fail because of several factors, you shoud post more logs.
If this doesn' t help, gain CLI access to your unit, and watch carefully the IKE debug (verbose level 2):
# set deb en
# diag deb app ike 2
you will find incoming packets, P1/P2 proposals, possibles timeouts etc..
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