Before that I decided to uninstall version 5.6 and one more time install 5.4.
I typed the configuration one more time and this time it started working properly. I was pretty sure the configuration was always the same, because the only thing I could do wrong was credentials, but in this case I would not be able to connect. Strange ...
If the config on the FG for IPSec is wrong, all clients would fail not only one particular. Every time you upgrade/downgrade firmware I would backup the entire config so that you can "diff" when you come back to the same major version of the firmware whatever the reason is.
I would still suggest check difference between FortiClient config on 5.6.0 and 5.4. Config. they should be the same except some new standard features like vulnerability scan, which I disabled because I haven't learned what it exactly does yet. By the way, 5.4 FC is significantly slower than 5.6.0 or 5.2 based on our SSL VPN performance test. Its improvement was mentioned in 5.6.0 release notes. We haven't tested with IPSec VPN though.
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