Make sure you are using the latest version of the eBay app for your operating system.
Try restarting your device between uninstalling and reinstalling to ensure a clean install.
For more detailed app help, try posting on one of the app boards:
https://community.ebay.com/t5/Mobile-App-Android/bd-p/androidapp
https://community.ebay.com/t5/Mobile-App-iOS/bd-p/iosapp
If you cannot get the app to work, try using a browser to go to ebay.co.uk instead to read your messages.
