Jira TestRail Integration Not Displaying

[Caleb Nelsen]

Setup: 

• Jira Server v7.4.2
• TestRail v5.2.0.3452

We use Jira Server on-premise with TestRail plugin to pull Runs/Cases into Jira. The plugin appears to have stopped working and displays only the Log in to TestRail button with an error above it. This is affecting everyone in our organization who uses this plugin. 

Error:

• You are not yet logged in to TestRail. Please log in to use the integration.

The problem occurs in any browser: FireFox, Chrome, Brave, Safari, etc.

I've seen this post which I believe is the same problem: https://community.atlassian.com/t5/Marketplace-Apps-Integrations/Jira-no-longer-shows-Test-cases-says-You-are-not-yet-logged-in/qaq-p/1402286

I've attempted the temporary fix by adjusting site-to-site but it doesn't seem to work. 

I also looked in the TestRail System Logs and I found this:

Does anyone have any suggestions I can try please?