I recently had the problem, that the IPMI virtual console was not working on one of my servers.

After searching the problem in icedtea I discovered errors while downloading files:

...
JAR https://localhost:443/liblinux_x86_64.jar not found. Continuing.
...
JAR https://localhost:443/iKVM.jar not found. Continuing.
...

I missed that the console works on other ports than 80 and that was the only forwarded port. So I changed my ssh command line to forward all IPMI ports:

sudo ssh -L 80:192.168.205.115:80 -L 443:192.168.205.115:443 \
  -L 623:192.168.205.115:623 -L 5900:192.168.205.115:5900 root@example.com