Because the extra layer of obtaining and displaying data in the Modern UX is different, the error message is generic.
More details on the reason for the error can be found in the app-ca logs. An example of an error would be:
ERROR 2019-02-05 09:38:59,940 [http-nio-14001-exec-6] rest.validation (clarity:admin:PPM_REST_API) ExceptionInfo :: Could not update resource. Resource name: timeEntries. Error code: timeadmin.timeentry.api.TASK_NOT_OPEN_FOR_TE Error message: TMA-1026: The task is not open for time entry.