How to troubleshoot email open rate being 0%?

I am seeing the open events in Mailguns UI. So seems they are getting through and opened ok.
Just some communication back to Ghost isn’t happening?

I see quite a few Fetched 0 events and aggregated stats for 0 emails in 236ms

EDIT:
Not sure if these logs help? Seems like there is some kind of error happening:

{"name":"Log","hostname":"ghost-travel-fodder","pid":1754,"level":50,"err":{"domain":"https://travelfodder.com","statusCode":401,"message":"Invalid private key","stack":"Error: Invalid private key\n    at IncomingMessage.<anonymous> (/var/www/ghost/versions/4.1.2/node_modules/mailgun-js/lib/request.js:327:17)\n    at IncomingMessage.emit (events.js:326:22)\n    at endReadableNT (_stream_readable.js:1241:12)\n    at processTicksAndRejections (internal/process/task_queues.js:84:21)"},"msg":"Invalid private key","time":"2021-03-30T10:32:12.643Z","v":0}
{"name":"Log","hostname":"ghost-travel-fodder","pid":1754,"level":30,"msg":"Capturing error for worker during execution of job: email-analytics-fetch-latest","time":"2021-03-30T10:32:12.656Z","v":0}
{"name":"Log","hostname":"ghost-travel-fodder","pid":1754,"level":50,"err":{"domain":"https://travelfodder.com","message":"Worker for job \"email-analytics-fetch-latest\" exited with code 1","stack":"Error: Worker for job \"email-analytics-fetch-latest\" exited with code 1\n    at Worker.<anonymous> (/var/www/ghost/versions/4.1.2/node_modules/bree/lib/index.js:385:40)\n    at Worker.emit (events.js:314:20)\n    at Worker.EventEmitter.emit (domain.js:483:12)\n    at Worker.[kOnExit] (internal/worker.js:231:10)\n    at Worker.<computed>.onexit (internal/worker.js:165:20)"},"msg":"Worker for job \"email-analytics-fetch-latest\" exited with code 1","time":"2021-03-30T10:32:12.657Z","v":0}