RDS services hosted on our windows server 2016 (Hyper-V virtual machine). We use default installation without any clustering/load balancing/etc - merely RDS services and licensing server.
Problem occurs when user count crosses 70-80 sessions (30-40 disconnected sessions and other live users) - start menu doesn't open, new users can't login (new = never logged before) and etc.
First thing we check was our resources - RAM, CPU, disk usage, networking, but all is ok, furthermore, resources utilization very far from 100%. There is no errors in event log.
Also while start menu just doesn't open, explorer works fine: users can navigate through directories and execute programs.
How to fix or debug this problem?