Hi,
This can happen sometimes after some important metadata changes, big versioning system synchronizations, after upgrading the Designer, or with unstable (nightly) versions.
Usually, this can be solved by one of these solutions:
- Rebuild cache (from the Impact view)
- selecting the mappings + right click / build / advanced / Rebuild metadata and process
- project menu / Clean projects
- open the mapping, make a modification (change an expression, not just the diagram layout), and save
If this is does not help, please contact the support team with a ZIP archive of your workspace and your exact version of the Designer.