Hi,
I have the same problem and nailed it down to a problem on the c# side.
I was switching between OpenAI and Azure OpenAI back and forth und figured out that the problem was introduced with Azure.AI.OpenAI Nuget package 2.1.0. I have an older test-project using Azure.AI.OpenAI 2.0.0-beta.5 and here the GetAssistantClient and GetVectorStoreClient functions are still working.
So it's not dependend on the server configuration in Azure AI Foundry.