Why can't I login to Cachly with my geocaching.com username/password when it works on the website?
Does your username have an apostrophe in it?
Since iOS 11 the keyboard started using "smart quotes" that are angled rather than the old "straight quote" by default. To use the old straight quote press and hold on the apostrophe key on the keyboard and you will be shown some options, one of which is the straight quote. Tap this and it should then allow you to login. Alternatively, you can disable smart quotes on your iOS device entirely by going to
Settings > General > Keyboard and toggling the Smart Punctuation switch.