I'm not sure if it's clear, but this is FreeBSD complaining that the environment string is invalid (an entry is missing the '=' separating the name and value). It's probably worth looking into why that's happening if you are able; whether nyx/stem/python is somehow causing that, or if it's something wrong/weird with your machine.
The commit to address this will silence the error, but it still seems like something is wrong; all env-modifying calls should fail like this after this point, it seems like.
Agreed. I can't (since I don't run FreeBSD) but if somebody that does could suggest an alternate os.putenv command for that platform I'd love to include it in Nyx.