We had a $ in ours and it worked. I saw post about the special
characters also when I was doing my troubleshooting, but the PW was
assigned by the VPN server admin and we couldn't change it easily.
We had this same issue with a fresh & updated install of Windows Server
2016 with the (-12) error using FortiClient v5.6.6.1167. We tested the
login on a MacBook (High Sierra) and Win10 (latest updates) and was able
to use the same username/password/...