Hello, @Jakob Stagis,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that the text of a calendar invitation you receive in Outlook initially appears blank in the reading pane, but displays correctly when the invitation is opened in the calendar.
You have indeed tried the common troubleshooting methods in Outlook, if you still have trouble in solving your problem, I would like to ask you a few questions.
1.According to your description, it works fine on OWA, reinstall Outlook and the problem goes away, I suspect that there is something wrong with the configuration of your device, please test it by logging into your account on a different device, does the same problem occur?
2.Are the “several of the customer's users” you mentioned receiving conference invitations from the same person?
I hope you can reply me in your free time, it will help me to solve your problem as soon as possible.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Thank you for your support and understanding.
Best Wishes,
Alex Zhang