Can you try the following?
1. Clear your browser cache
Although the app is a standalone app on your mobile, it uses external services to securely log in. These services run through the browser you use by default on your phone, and therefore there may be something stored in the cache that prevents your login from working properly.
iPhone
Safari: The setting is found under Settings > Safari > Scroll down and find "Clear History and Website Data"
Chrome: Open the Chrome app, tap the 3-dot menu in the bottom right corner, select "History" and select "Clear browsing data"
For both, you can choose to clear the entire cache or just the compugol cache.
Android
Chrome: Open the Chrome app, tap the 3-dot menu in the bottom right corner, select "History" and select "Clear browsing data"
2. Check your Golfbox login at golf.dk
If you use Golfbox to log in and have problems, you should check that your login works by logging in to golf.dk with your member number and password.