Could not resolve value of Project Key
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'
The project key of one of the two projects which are being synced has been changed.
In order to resolve this problem please follow the instructions on renaming a project which are described in this knowledge base article.