Error in the RFC communication: &1 (&2)

What causes this issue?

A service of the TREX search engine was called. An RFC connection to the
gateway is required for this, but the connection could not be
established. As a result, the TREX services cannot currently be used.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Contact your system administrator.

Procedure for System Administrators

Check the RFC connection "&V2&" using transaction SM59 (TCP/IP
communication).
Error occurred during RFC connection: "&V1&"

Error message extract from SAP system. Copyright SAP SE.