My understanding (having read some random stuff online) is that it comes and goes in waves. Like, half of an office can log in and work, but the other half can’t. So its some sort of cascading effect. So, maybe some people will be able to use slack intermittently. I still get the “can’t get in” message and it just oscilates between that and trying to connect. The closer you are to california the better off you are apparently.
Bandwidth & ports are ok according to diagnostics so its either something wrong in the network (DNS issues) or its something on their servers and their application has hit some critical bug/hardware failure.