This error might be caused by changing this setting.
When having this error in OWA, ECP or Outlook:
You can fix this by setting the correct time zone in Outlook (you cannot change it through ECP — you need Outlook):
File > Options > Calendar > Time Zone section
Label: YourCity (e.g. Brussels) Time Zone: YourTimeZone (e.g. UTC+1 Brussels).
Be sure to pick the correct time zone and fill in a label.