Websharper doing async call returns server error 500

I'm trying to do async db query. The server side function looks like this

[Rpc]
let dbAsyncCall () =
    async {
        return! query {
            // ...
        } |> Seq.executeQueryAsync
    } 
    |> Async.StartAsTask 
    |> Async.AwaitTask 

On the client side I call it like this

async {
    let! data = Server.dbAsyncCall ()
    // ...
}

When the code gets executed I receive the following error

What's going on?

Thanks!