Label for &1 &2 contains invalid character '&3'

What causes this issue?

The description contains an invalid character. Valid characters include
numbers from 0 to 9, letters from A to Z, and the underscore '_'.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.