Some users may get an error like "Error: Loading Calendar Service" although it worked before. This is due to a corrupted Symbian configuration file in C:\Private\2000f847. In order to get rid of the error, you will have to reset your phone. Do not restore files from a backup, or you will restore the broken config too. An alternative is to replace the corrupted config file manually, but you will have to hack your phone in order to have access to it. You can find the non-corrupted version of that config here. More info on this in the bug report.
Update: Another solution has been found which does not require a device reset or a hacked firmware: if you're are lucky and made a backup before the issue occured, you can use it to restore the device settings from that backup to get this widget working again.
This problem was introduced by a Nokia firmware update. The new firmware breaks the ability for homescreen widgets to access your calendar. I've informed Nokia about this issue and developers confirmed. It will probably be solved in an upcoming firmware update. In the mean time there is nothing you can do to get it working again.
The following devices are affected by this bug:
If you have a lot of events in your calendars, the widget might be every slow when updating. Try to decrease the "month range" setting. That way, the widget will load less events and refresh faster. Also turning off the display of ToDo notes can improve speed if you have a lot of them.
To make the widget background transparent, follow these steps:
This is a bug in Nokia's WRT library (the status of a ToDo event is sometimes is reported as "undefined"). There is nothing the developers can do about this. Wait until Nokia releases an updated firmware with this bug fixed.