License Key Error displaying on Calendar

[Kirt]

My calendar is suddenly displaying this message:

License Key Error.
[Running in limited mode] [Help]

From this page I can't identify any reason why this would suddenly start showing up.

• A trial version license key was used and the trial period has ended.
  Negative, we're running a full license version 6.3.23
• You've recently upgraded to a newer version.
  Negative, auto update is not yet enabled.
• The license key information was entered incorrectly.
  Negative, license key has not been altered recently.
• The domain name or server IP address has changed.
  Negative, we're still using the same IP address that was registered with the license key.

From the Calendar Admin panel, no error message is being displayed. It only shows on the webpage that contains the calendar.

Help please?

[Brian]

Another option (which has since been added to that page) is:

.:: The configuration of PHP on your server has changed.
The licensed key depends on some functions of PHP to work correctly. A change in the features enabled in PHP could cause the license key to be disabled (this could be do to a recent upgrade of PHP). If the license key needs to be updated because of this, please submit a License Reset Request.

Either way, you would still need to use the form to request a new license key.