The setInterval() timer will not trigger on devices using browser <=7.1.17501, when the widget is moved into the background, when the screen is locked, or when the device is locked. This behavior is in conflict with the older WRT versions.
Note the special case with home screen widgets: On a device that supports home screen widgets (for example, Nokia N97 and N97 Mini), the timers of all running widgets are stopped if there is at least one widget in the home screen. If you want to enable the timers, please make sure that there are no widgets in the home screen. This is intended behavior for power saving reasons.
Fixed in upcoming firmware updates. However, the fix does not apply to cases where one or more widgets are running in the home screen.