Existing epics/tasks cannot be opened, and a new task/epic also cannot be opened after they're created. I see the following error that I've never seen before. Nothing has changed with our project; permissions are as they were.

We couldn't connect to that work item.

Make sure that this work item actually exists in that space. If it does, try again in a few minutes. If you still can't link to the work item, contact your Jira admin.