Integration between Jira and TestRail have worked fine in both directions for many months, but now when I view the "TestRail: Runs" field in Jira, it says "<companysubdomain>.testrail.com refused to connect"

So I created a new API key through Atlassian and used it in TestRail to reconfigure integration, but the issue remains.
Integration is working from TestRail to Jira, just not from Jira to TestRail.