I take that back, I am getting some errors now. Any tasks that are not completed in less than 24 hours after they are downloaded are invalidated with the error "Completed, too late to validate", even though the task return deadline is actually seven days and not just one day.

If you are going to invalidate tasks after a single day, you need to change the deadline of the tasks to one day so the BOINC Manager doesn't download more tasks than can be completed in that day. Either that, or let tasks be returned and validated any time within the current seven day deadline.

https://boinc.termit.me/adsl/results.php?userid=26&offset=0&show_names=0&state=5&appid=