2022-12-07 16:28:56.117964 ike 0:VPN:14172: sent IKE msg (RETRANSMIT_SA_INIT): 188.8.131.52:500->0.0.0.:500, len=400, vrf=0, id=ea6a7b66ed952355/0000000000000000
2022-12-07 16:29:05.087800 ike 0:VPN:14172: negotiation timeout, deleting
2022-12-07 16:29:05.087957 ike 0:VPN: connection expiring due to phase1 down
Typically indicate that the remote side stopped responding to the FortiGate's attempt to re-negotiate the IPSec tunnel.
If this was working before but no longer is, what changed? Was a firmware update performed? Was there a modification in the network? Did your cloud hosted server switch providers?
Since you already know how to enable the debugs for the ike process, it would be best to keep that enabled and periodically check for when the issue is occurring so you can gather information that may uncover the cause of this issue. However, based on what you have provided thus far, it just looks like the remote side stops responding.
I know this is not much but I hope this helps. If you can provide anymore information, we can try to dig a little deeper.