Check that you have installed the latest Nokia Store client version. Check for any updates in SW Update menu of your phone or accept the upgrade request if prompted when starting the Nokia Store client.
Nokia Store client may fail to start also if you have installed special fonts in your device.
To check if there's additional fonts installed in your device, start File mgr. (Menu > Applications > Office > File mgr.) and check if folder Resource\Fonts can be found on C:Phone memory, E: Mass memory or F: Memory card.
To remove the special fonts, uninstall the application which has installed the fonts. Select Menu > Settings > Application manager > Installed apps, long press the application name and select Uninstall in the pop-up menu. If it is not clear which application has installed the fonts, you can view a list of the recently installed applications in Options > View log.
Alternatively, if no font application can be found in Application manager, remove the folder Resource\Fonts and restart the device.