Do not use non-7-bit ASCII characters (see long text)

What causes this issue?

An error occurred when the HTML text was converted. The source code
probably contains non-7-bit ASCII characters. These characters can cause
problems in the customer system (in combination with the system code
page).
The error occurred when the the following template was edited: &V1& &V2&
&V3&.

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?

Only use 7-bit ASCII characters, even in comments.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.