Welcome to the forum!
What applications that you are unable to open aside from the Oxford Mini Dictionary? By the way, is the Oxford Mini Dictionary a pre-installed application or downloaded from the Nokia Store? Are you getting this issue with the pre-installed applications only? How about the downloaded one? For the downloaded applications, you can try uninstalling and reinstalling it. Tap and hold the application, tap "x" button to uninstall it and the monitor your phone.
Note: Make sure you have downloaded the applications from the Nokia Store to ensure that it is compatible with your Asha 501.