Hi IC,
These are the actions that I'd think of doing:
– uninstall and install the app again
– think about recent changes in your env. that could affect the functionality of the app (ie. Microsoft's updates. other 3rd party app that could change the env and/or the web app)
– try different machines/browsers
– iisreset the server
– check teh ULS log file for any related entries
Please let me know how it goes.