Malformed or invalid time zone in working hours XML in mailbox

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.