Here's how the problem happens:
The router on the client end sends an HTTP request over TCP/IP (the Vault login) to the web server, which responds with 1500 byte packets. 1500 is bigger than the router is configured to accept, so it sends back an ICMP packet that tells the web server to start sending smaller packets (1492 bytes). Since the web server never sees the ICMP message, it continues sending 1500 byte packets that the router discards.
Possible fixes include:
- Configure your router to accept 1500 byte MTU sizes and/or defragment packets
- Configure your server and any firewalls to allow the correct ICMP traffic to and from your server
It's also possible for the issue to be solely on the Windows client. Use netsh to change the MTU and reboot the client.