For one day (yesterday) the “overview with graph” shows 0 visits, while all other widgets look normal. The Log shows visitors that day.
The page “Visitors->Overview” also shows 0.
All other days, including today, the “overview with graph” shows valid looking data.
What could be the problem and how can this be solved?
This bug is expected but should not happen again after Piwik 1.6 to be released in a few days. In the meantime, this is a known bug (happening rarely especially if you use How to Set up Auto-Archiving of Your Reports - Analytics Platform - Matomo )
It never happened before.
Is it possible to let it recalculate the results of that day? (now or after Piwik1.6?)
How can we identify when this bug happened? Does the outcome always have to be 0, or can it also show a different wrong number? When is there a reason to doubt the Overview Graph?
I have the same problem in the Last visit graph widgets, all normal, but two days with 0. I’m in piwik 1.6 
If I see the Visitors in real time for these days I can see all the real visitors not 0
Make sure that “Allow browser archiving” is enabled in Settings > General settings
Yes, it’s enabled, but one day is at 0 yet, other are ok
I also still see the problem in 1.6 and I also have “Allow browser archiving” enabled
Thanks for the report. Please see ticket: Some days incorrectly show 0 zero visits when archiving was running the first second of the day · Issue #2728 · matomo-org/matomo · GitHub
Also, it would be great if you could post there the information to help assess if you experience the same bug. In particular, can you look at the piwik_archive_numeric table and note the datetime of ts_archived like the bug reporter did?
please post your comments in the ticket, not here. Thanks!