Thank you very much! 
I was able to get Slack working again by replacing slack with (slack.override { nss = pkgs.nss_3_44; }) in environment.systemPackages, following this tip.
Thank you very much! I was able to get Slack working again by replacing slack with (slack.override { nss = pkgs.nss_3_44; }) in environment.systemPackages, following this tip.