Hi Chris,

Just to report I've had this error appear in my experiment. It affected subjects in the transition between a page using GroupByArrivalTime with a custom method and the next page. 

Subjects somehow managed to pass the GroupByArrivalTime as if they were in a group but they were all alone in their group. Of course the experiment crashed.

I have everything on Sentry if you want.

Error occurs in otree/tasks.py

```
def get(url):
    try:
        request.urlopen(url)
    # some users were reporting URLError but not clear what URL it was
    except URLError as exc:
        raise Exception(f'Error occurred when opening {url}: {repr(exc)}') from None
```

Indeed some users are reporting this error!

—Geoffrey