site-to-site VPN

I have 2 cites and both sides has astaro v7.
Bothsides:
firware version:7.512
AES-256
Local RSA key

I configured on the astaro city one(HQ)site-to-site VPN and the other City(our branch-1) too.

last 2 years connections between HQ and our branch-1 was like a charm and no problem.

last week bettween two sites started lose connection.
connections for 2 hour or less establish and works without problem.But sometimes prox.30 minutes lose connections the both site.
when lose connections then i look HQ astaro admin page->site-to-site VPN
site-to-site VPN our_Branch-1the status RED. After restart HQ astaro then again GREEN.
can you tell me please solution?
Thanks