This is typically caused when connecting to intermediate servers like an MDEX Load Balancer (VIP) . To confirm that the issue is with VIP, go ahead and update the assemblerApplicationConfiguration of one of your page servers to point directly to the MDEX instead of VIP.
If this error goes away, we know there is an issue with the VIP requests. Work with the server administrator to fix the VIP configurations.
Typical issues are –
VIP is not setup/enabled to use HTTPs
The Health monitor for MDEX is not setup correctly in VIP