Fortigate VPN client “Unable to logon to the server. Your username or password may not be properly configured for this connection. (-12)”

If you use SSL based VPN from Fortinet, you can use openfortivpn software which is part of Ubuntu and Fedora.

$ sudo apt install -y openfortivpn || yum install -y openfortivpn
$ touch openfortivpn.conf
$ chmod go= openfortivpn.conf

Edit openfortivpn.conf:

host =
port =
username =
password =
# trusted-cert =

Connect the first time:

$ sudo openfortivpn -c openfortivpn.conf
ERROR:  Gateway certificate validation failed, and the certificate digest in not in the local whitelist. If you trust it, rerun with:
ERROR:      --trusted-cert  <some-random-string-to-add-to-trusted-cert>

Edit openfortivpn.conf file and update trusted-cert option with the string from the error. Make sure the option is not commented (remove the #).

Each time you need to connect, run:

sudo openfortivpn -c openfortivpn.conf

You could also do it straight through the command line and make an alias for it in your .bashrc. This is less secure as any user on the system will be able to see the password using ps.

sudo openfortivpn -u <USER> -p <PASSWORD> --trusted-cert <CERTIFICATE> <SERVER>:<PORT>

To disconnect press: Ctrl+C

Try to run the CacheCleanAP.exe app that locates in the same folder that FortiSSLVPNclient.exe is. By default, it is in the c:\Program Files (x86)\Fortinet\SslvpnClient\ folder.
Although in a few moments after starting CacheCleanAP.exe, the Windows app crash error appears, it helped me to resolve the issue that FortiClient connects to the VPN but the connection didn't work.

I've been forced to use FortiClient for connecting to a partner's VPN in my workplace for about 3 years and I can say it is a bit buggy app especially if your Internet connection isn't perfect.
Sometimes FortiClient works great, but sometimes it can't connect without any observable reason or connects but the connection doesn't work.
The main disadvantage of FortiSSLVPNclient is that it changes routing table, so packets to many destinations that isn't supposed to go through VPN, go exactly through it.
Initially I worked with the 4 version of FortiClient installed, after that I tried version 5 but this didn't help to resolve any problems. Moreover I was forced to install a bunch of unneeded apps (antivirus, etc.) that comes in the version 5. So as a result, I removed version 5 and returned to version 4.

A few days ago I couldn't connect to the VPN 1 or 2 days in a row. But yesterday I found a solution that did the trick. Running CacheCleanAP.exe immediately resolved the issue and I'm able to work again :)

Hope this helps.