Visits Over Time Count Disappeared for a Day

For some reason a day in the “Visits Over Time” graph is gone missing and shows 0 visits during that day – while all other days are working fine.

Piwik is still storing visitor information from that day, just the visitor count is gone/broken somehow.

Any ideas how to fix this?

Screenshot from Piwik:

Same applies to the static image graph where I see a dent in the number of daily visits: