Could not resolve value of Project Key

Symptoms

An issue does not get synchronized and appears in the troubleshooting section with the category Could not resolve value of Project Key and a message containing, e.g. The value 'PROJECT-KEY' does not exist for the field 'project'

Cause

The project key of one of the two projects which are being synced has been changed.

Resolution

In order to resolve this problem please follow the instructions on renaming a project which are described in this knowledge base article.