No connection to a SSAS database with individual credentials

Hi,

I'm trying to connect to a SSAS database (MS SQL server) with Power BI Desktop. After selecting:

1. "Data Source" -> "SSAS"
2. filling out the server address and selecting "Import"
3. and providing the credentials in "Windows" -> "Use alternate credentials"

the connection fails with the error "Object reference not set to an instance of an object.". This happens with the version 2.50.4859.782 from September 2017 and on different machines and networks.

However, I tried using the older version 2.45.4704.442 from April 2017 of Power BI Desktop and here everything works fine. Also, establishing a connection to this server from Excel and other XMLA tools works as expected.

After debugging the network traffic with Wireshark I noticed, that there is absolutely no communication from Power BI (September version) to the outside when using "Use alternate credentials" or Basic Authentication. Only when "Windows"/"Use my current credentials" is select, Power BI is speaking with the server (the login works here).

I don't know where to start to fix this. It seems that this is not a problem of the SSAS server itself. Perhaps I miss a settings change in Power BI, between the April and September version. But i can't find anything in the public change logs for the versions since April.