VPN not working

Hi,

on Friday I was able to connect to VPN. Today It's not possible anymore, because of the following error in FortiClient (latest version): "IPsec-SA request for xxx.xxx.xxx.xxx[500] queued due to no phase1 found.".

The configuration hasn't been changed for several days or weeks. I have already rebooted the FortiGate 60D which didn't fix this issue. Already compared the config with an older one. There is no difference except different SSL certificates, but we also have this issue with IPSec.

Any ideas for a cause?