Blocked IPs over VPN

[mediaadventure]

Hello,

we have a vpn-tunnel between a fortigate 80D and a 30E. On the 80D site we have a win-Server for files/AD/Exchange. On the 30E the fortigate makes DHCP for 4 PCs. All works fine for several days. But than the first pc on the 30e site has no Access to the 80D site (Server/files) and so on. I give the pc a static ip outside the dhcp-range and all works fine again. After some days the same Problem. I changed the static ip again and it works again. IT seems something blocked the ip after some Actions/days. What can this be? What filter? Where i can find it in Protokolls? IPS?

Thanks, Klaus