Hi Funke,
I'm sorry to hear that you're having trouble with your Upwork Time Tracker App. Please check this help article for troubleshooting steps.
Also, please try using a third-party uninstaller instead of just simply uninstalling it from your program uninstaller. Whenever you uninstall and reinstall the App, there are bits of data left that may interfere with the newly downloaded version. There are third-party uninstallers that you can use to completely remove all the data left from the former versions of the App. After doing so, download the App again and install it. If this still doesn't help, please let us know. Thanks!
~ Joanne