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: -L 443: \
  -L 623: -L 5900: root@example.com