Apps have a tendency to start misbehaving after major Android system upgrades like Marshmallow. I had a number of apps start acting out of line after installing various versions of the Nexus 5 Android Lollipop update. Marshmallow isn't Lollipop though.
While some people have encountered issues with their apps, my applications have been holding up extremely well. I've seen a few crashes (Twitter, Facebook, Gmail, Chrome, Spotify) but that's nothing out of the ordinary. I can deal with a crash here and there.
I've also installed every single bug fix update that developers have thrown at me so that could explain my good luck during these two weeks with Marshmallow. If you do run into problems with your apps, bug fix updates are the first place to turn.
If that doesn't work, try reinstalling the app or clearing the cache. And if neither of those work, contact the developer. Developers of apps like Slack love user feedback so don't shy away.
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