I think this may be fixed in snapd 2.63 which is now in beta. Try snap refresh --beta snapd and please report back whether slack now starts correctly or not.
This didn’t work for me directly because I didn’t have snapd installed via snap. So I first had to install snapd, then I could switch the beta channel. And then it worked.
IMHO, this is critical enough that it should be backported to the current stable version or released quickly. It seems to effect several snaps and especially business oriented ones which made for a more stressful Monday morning.