COMException: A custom field with this name but a different data type already exists.

First, install the Redmine Shared API

It allows the Outlook Add-in works correctly with Custom Fields.

Second, please delete the following views: Issues, Issues Detailed.

Go to folder My Issues -> Home tab -> Current View block -> Manage Views

Open

Then please run Reset Data Store. Go to Redmine tab -> Settings -> Storage -> Click Reset Data Store.