After the development release on the 2nd of May, locations where achievements are embedded into content will display "Invalid GUID".
This is because of a change in how achievements are embedded into content, from using their unique identifier to a 'template' that allows administrators to search for the achievement. Unfortunately a mishap with the logic meant that it ignores where a GUID was put in the field, and results in this error.
There should be no issue with gaining the achievement, a hotfix is being worked upon, else it will be fixed in the next development website release.