I have developed a simple VSTO Add-In for Outlook which connects to an SharePoint on-premise server. The issue is that for some users the ribbon with the data doesn't show. For others it works just fine. For those where it doesn't work, they can disable a random add-in, then my VSTO add-in shows up until outlook is restarted, then it is gone again.
I have read several articles advising to disable all other add-ins which is not an option for me. It sounds to me that it could be an add-in conflict. The only solution I could find was an explanation that the only thing to be done was a professional-level service request with Microsoft.
I have tried rebuilding my entire solution with the exact same results. The solution is written in C#.
Please help.
You need to find out what is the exact error. For that you need to configure the host application to report about any UI erros.
By default, if a VSTO Add-in attempts to manipulate the Microsoft Office user interface (UI) and fails, no error message is displayed. However, you can configure Microsoft Office applications to display messages for errors that relate to the UI. You can use these messages to help determine why a custom ribbon does not appear, or why a ribbon appears but no controls appear. Read more about that in the How to: Show Add-in user interface errors article.