Outlook desktop (new Mac OS X UI): addFileAttachmentFromBase64Async throws code: 5001, message: "An internal error has occurred." in pinned add-in

[guzman-rc]

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Provide required information needed to triage your issue

In an Outlook add-in (new Mac UI preview) for compose mode, we are using the addFileAttachmentFromBase64Async method to add attachments. In Outlook desktop "new Mac UI preview", the "Internal Error" occurs if the add-in's pane is pinned.

In Mac Outlook desktop "Classic UI" this method work correctly.

Your Environment

• Platform [PC desktop, Mac, iOS, Office on the web]: Mac
• Host [Excel, Word, PowerPoint, etc.]: Outlook
• Office version number: Microsoft Outlook for Office 365 MSO version 16.65 (22091101)
• Operating System: Monterey (v.12.6)

Expected behavior

Current behavior

Steps to reproduce

1. Install MetaClean for Outlook from Microsoft App Source (https://appsource.microsoft.com/en-us/product/office/WA200003633)
2. Start Outlook and select new Mac UI for Outlook.
3. Create a new email in Outlook desktop.
4. Click "MetaClean for Metadata" button.
5. Click on the add-in’s pane and select a file to attach.
6. The code that causes the error is (See Screenshots from Web Inspector):

Link to live example(s)

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Provide additional details

1. In Mac Outlook desktop "Classic UI" this AddIn work correctly.

Context

This bug is causing complaints from our customers.

Useful logs

• [ ] Screenshots from Web Inspector Enable new Mac UI preview:

Error returned by addFileAttachmentFromBase64Async method:

[exextoc]

We have a repro of this issue and are working on it. We unfortunately have no timelines to share at this point