Thank you for the report – seems to be a bug in Mylyn we may have inadvertently picked up. We’re going to include a fix to this in an upcoming MyEclipse maintenance release. In the meanwhile, there are a couple of workarounds.
1) You could update to a newer version of Mylyn by installing it from this update site: https://download.eclipse.org/mylyn/updates/release/3.26.1
2) Delete the directory [workspace_location]/.metadata/.mylyn/.taskListIndex/ before starting MyEclipse. This will result in the Index being rebuilt and resolve the initial error you see. You will still see errors when you save / edit existing tasks or create new tasks, but I believe they are still persisted by the IDE.
Sorry for the inconvenience caused, thanks again for the report!