Failed to install Outlook Web Add In as a Developer

Question

Hello,

when trying to install my simple custom add-in for Outlook Web I get the following meaningless error message:

"Installation fehlgeschlagen

Wir können diesen Vorgang im Moment leider nicht abschließen. Versuchen Sie es bitte später noch mal."

The add-in I want to install is a example from microsoft itself.

I managed to install my own add-ins before (you can see them on the bottom of the screenshot), I followed this guide exactly, I chose the variant for Visual Studio:

https://learn.microsoft.com/en-us/office/dev/add-ins/quickstarts/outlook-quickstart?tabs=visualstudio

Add-Ins that I have already installed before, I have uninstalled and when installing the same Add-In again I get the same error message.

I have the behavior after clearing cookies and in two different browsers (Firefox and Edge).

On another mail account, the same message comes up when installing an Outlook web add-in from Visual Studio.

I found the following sites, however none were helpful:

https://learn.microsoft.com/en-us/office/dev/add-ins/testing/troubleshoot-development-errors

https://learn.microsoft.com/en-us/office/dev/add-ins/testing/testing-and-troubleshooting#add-in-doesnt-activate-in-office

https://learn.microsoft.com/en-us/answers/questions/475683/cannot-install-outlook-add-ins

https://learn.microsoft.com/en-us/answers/questions/1354256/cannot-install-outlook-addin-on-windows-for-person

https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-add-ins-fail-to-install/b26f0981-7eaa-40a0-a9a2-7aeae2b521a8

Does anyone have any advice on how to solve this problem?

Any help is appreciated.