Connection to SAP Web IDE could not be established (see long text)

What causes this issue?

Communication error: Connection to SAP Web IDE (di-core) could not be
established.

System Response

The system displays an error message.
You can find the cause of the communication error in the following error
messages.

How to fix this error?

Read the following error messages to find out more about the
communication error.
In general, check in transaction SM59 that Host and Port are maintained
correctly for destination XSA_DEVX (default name of destination to SAP
Web IDE). In addition, check that the connection test for this
destination returns a logon prompt. Note, however, that the logon prompt
cannot be used because the OAuth protocol is not supported in
transaction SM59.
For more information about this destination, see the section
<LS>Creating Destinations in ABAP</> in the configuration guide <LS>
Configure your ABAP Development System for Development of HDI Objects
</>. The guide is linked in SAP Note 2569651
(https://launchpad.support.sap.com/#/notes/2569651).
If you do not use XSA_DEVX as the default name for the destination to
SAP Web IDE, you find the name of the destination used in the system
using transaction <LS>SCTS_HTA_TOOLS -> Define Parameters of HTA-for-HDI
Configuration</>. The value of the <LS>Destination for DevX</> parameter
is the name of the destination that HTA uses to connect to SAP Web IDE.
Note: If transaction SCTS_HTA_TOOLS does not exist in your system, you
can use transaction SA38 to execute program SCTS_HTA_CONFIGURATION to
find out the name used for the destination to SAP Web IDE.
If this doesn?t solve the issue, check the troubleshooting sections in
the configuration guide or in SAP Note 2569651 for further information
about how to proceed when errors occur.

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

Error message extract from SAP system. Copyright SAP SE.