If you have too many folders in the root folder in inventory, it may cause login timeout in the viewer: request to https://login.agni.lindenlab.com/cgi-bin/login.cgi with inventory-skeleton option returns too much data (1.6Mb and 3121 folders in my case), and sometimes it responds very slowly (more than 40 seconds - default login timeout in the viewer).
How did I know that? I intercepted request using mitmproxy and tried the same request using curl.