VPN going down - No route to host

We have a site to site VPN set up between two of our sites. Both sites are using UTMs. The VPN has worked perfectly for months. No changes have been made.

In the last couple of weeks the VPN has dropped four times for less than a minute. The error logged is:
/var/log/ipsec.log:2019:08:12-00:38:12 perimeter1-1 pluto[7754]: ERROR: asynchronous network error report on eth6 for message to port 500, complainant No route to host [errno 113, origin ICMP type 3 code 1 (not authenticated)]

Our ISP hasn't found anything, but I don't trust them.

Can anyone tell me what's going on?