Windows 10: Add account to mail app doesn't work

[German]The mail and calendar app that's shipped with Windows 10 tends to fail. In my case, I was facing the situation, that no more mail accounts can be added, removed or viewed. Here is some information including a possible solution.

The problem description

I was recently affected by this problem myself when I wanted to update a small beginner's book on Windows 10. In the section about setting up the mail app I was suddenly faced with the problem that it didn't work on Windows 10 20H2. For example, when I went to Settings – Manage Accounts – Add Account from the app's sidebar, nothing worked when I selected the command. Here is a German screenshot of the popup window.

It was also not possible to select an existing account to view the configuration, nor to delete an account. A quick internet research showed me that this problem has been running through the mail app for years. This thread in the Microsoft Answers Forum is from 2015 – we are writing in 2020 now.

Cannot add accounts to mail and calendar on windows 10

I've been trying to add an account to my windows 10 mail app but it doesn't work.  I try deleting it and reinstalling and still cannot add any accounts to mail or calendar app.  The add account button does not do anything or show any error messages.

My machine is used for tests only, and has been updates to Windows 10 20H2 recently. There is no third party antivirus software and only Microsoft Office 2019 Pro Plus installed.

First repair attempts unsuccessful

I then tried to repair the app by running wsreset.exe (but only resets the store). Then I went to Settings -> Apps -> Apps & Features, selected the app, and then clicked on the Advanced Options command. There is a Reset button to reset the app to the factory default settings. Alone it did not help.

The next attempt was to uninstall the app and then reinstall it via the Microsoft Store. The first time I started the app, the window for setting up a mail account appeared (picture above). But the Add account option was dead, nothing happened when clicking it. So I was exactly as far as before.

This repair method helped

In the US Microsoft Answers Forum you can find this thread from 2015, I mentioned above. The poster wasn't able to manage (add, change, delete) accounts with the Windows 10 Mail App. There Alexei Stukov gives the crucial hint that a capability OneCoreUAP.OneSync is missing. 

Here the construction of Windows 10 strikes mercilessly. On this page Microsoft writes that oneSync is a feature on demand, that is installed on demand. It is an e-mail, contact and calendar synchronization component. If this component is missing, the mail app doesn't work. This is what it says: If you don't include this feature on-demand on your Windows image, UWP apps such as email, people and calendars will not synchronize properly.

The missing feature can be added in an administrative command prompt / PowerShell console with the following command..

DISM.exe /Online /Add-Capability /CapabilityName:OneCoreUAP.OneSync~~~~0.0.1.0

I ran the command in the PowerShell console (see the following screen shot). The installation took quite a while – but afterwards managing accounts of the mail app worked as expected

While researching for OneCoreUAP.OneSync, I came across this German article (deleted) by a fellow sufferered the same, who had also tried all kinds of repair attempts before. That's what I like on this 'Windows as a surprise' (Waas) – ok, I'm just kidding. I never used that buggy Windows apps at all – and Windows 10 is only valid as a test machine for me.

The above episode again showed me how fragile the whole construction of this 'Windows as a surprise' actually is. I have no idea which (feature-)update removed the on-demand capability OneSync. But a normal user has hardly any chance to remedy this.