Thanks so much for the fantastic response!
Using the curl command I got the same error but an error in the server.log file of “IOException encountered while reading from a byte array input stream”.
Once I Googled that, I found a similar issue here on the community (Graylog Content Packs page will not load after upgrading from v3.0 to v3.1) which lead me down the path of deleting the mongodb document for the notification in question:
db.event_notifications.remove({"_id" : ObjectId("60d678791876621ab5471bff"), "title" : "Slack Notification"})
Once I was able to delete that the page loads fine and I’m able to recreate alerts.
Thanks so much for giving me the hint I needed!