Why IPsec Tunnel phase2 sporadically disconnect?

Hello,

we have an error in the IPsec Tunnel service. Individual subnets that are routed through the IPsec tunnel are sporadically disconnected. only deactivating and activating the tunnel fixes the error. Keep alive in IPsec Service is enabled. Sometimes connection from client in branch to remote subnet disconnect by any clients, sometimes the phase2 connection to remote subnet disconnect completely. This error occures in one branch. WAN interface without failures. IPsec service always enabled and active. IPsec tunnel config has been checked several times on both firewalls and is identical. Thanks for your support.