Outlook add-in doesn't open in shared mailboxes despite SupportsSharedFolders set to true

Here's what we do:

• We create an add-in with the Yeoman generator as described here.
• We modify the manifest.xml to support shared folders:

...
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
    <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
      <Requirements>
        <bt:Sets DefaultMinVersion="1.3">
          <bt:Set Name="Mailbox"/>
        </bt:Sets>
      </Requirements>
      <Hosts>
        <Host xsi:type="MailHost">
          <DesktopFormFactor>
            <SupportsSharedFolders>true</SupportsSharedFolders>
            <FunctionFile resid="Commands.Url"/>
            ...
  </VersionOverrides>
</VersionOverrides>

• We sideload the add-in in OWA for one user via manifest-file.
• We start a local development server with npm run dev-server.

The Add-in works perfectly in the user's own mailbox. But when we try to activate the add-in for a mail in a shared mailbox we get this pop-up-message:

"You cannot peform this action. This add-in scenario is not supported in this folder."

What do we have to do to get the add-in running in shared folders/mailboxes?

We're using Outlook-Client Version 2002 and Exchange 2016 15.01.1913.005. A call to Office.context.requirements.isSetSupported('Mailbox', '1.8') returns true.