You should open the file test-all-constraints.fet from FET (with the language set to your language) and check the all time
constraints and all space constraints dialogs, the description and detailed description for each constraint should look good.
Constraints should be different one from the other, for instance same starting time and same starting hour, or preferred
starting time (for a single activity), preferred starting times (for a single activity, for a set of activities and for a
set of subactivities) and preferred time slots (for a single activity, for a set of activities and for a set of subactivities).
Please make sure all these kinds of constraints are clearly separable one from each other.
This file is not solvable, it just contains all constraints.