some weeks ago I developed an Outlook Web Add-In. I uploaded the manifest to my Exchange server and everything was working fine. It got distributed to my Outlook Client and worked as intended. This week I wanted to try it for Outlook 2016, but the client is not loading it. Going back to my 2019 version I see, that it also disappeared there. As I am checking the Add-Ins in Exchange Admin Center I am getting this error when loading the list of Add-Ins: Method not found exception in EAC
Of course the Outlook clients also wouldn't get the Add-In, if the exchange server cannot process this.
The exchange management shell is throwing the same exception.
Does anyone know what to do?
It is an Exchange 2019 with the newest CU installed.
I opened the dll in the Exchange server folder and can clearly see, that this method exists. Then I replaced the dll, but to no avail. The only thing that was done to that machine was installing Wix Toolset and the .net 3.5 Features for that, but i reverted those changes already.
I found for example this: https://learn.microsoft.com/en-us/answers/questions/253128/exchange-2016-add-in-deployment with the same problem as I do, but the suggestions didn't help.