Consistent Console error message: invalid signature for com.cisco.kext.acsock I have Console open on my Mac (latest OS X El Capitan) and this error keeps reappearing a few times every second.

4/19/16 1:15:18.021 AM com.apple.kextd[47]: ERROR: invalid signature for com.cisco.kext.acsock, will not load

Does anyone know what exactly this means, and is it a bad thing? It keeps appearing at least a couple of times every second, so it's very persistent. I couldn't find much on this error.

Is it something that can be fixed easily?
I had the same issue. kextd was consistently using around 14% CPU and tailing of /var/log/system.log showed that it was trying and failing to load com.cisco.kext.acsock over and over again.

I uninstalled the Cisco AnyConnect VPN client using its installer caused the problem to end immediately.

I suspect I had an old version of AnyConnect installed that was from the pre-El Capitan days, and hence it didn't have a valid the signature. Luckily I don't need the AnyConnect client (I had installed it a while back briefly for some troubleshooting) since I use the built-in client.