J'ai trouvé ce qui suit dans
http://www.cgeo.org/faq#login-failed" onclick="window.open(this.href);return false;
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First of all please check carefully whether you entered the correct username and password. You can find this in:
Menu → Settings → Services
Afterwards select “Check authentication” (for geocaching.com users only) to see if login is either now possible or what error is returned.
If you have double-checked your credentials and still get an error message (e.g. Login failed, Unknown Error) the following might be helpful:
Please login on the geocaching website (using your smartphone browser or a PC) and check that your account is not deactivated and your mail address is validated on the website.
Please check that your phone is working with the correct date and time. If the date is wrong this might lead to problems establishing a secure connection to the server.
Sometimes network providers insert messages into the HTML-traffic which prevents c:geo from logging in while on mobile network. A clear indication for this problem is, that c:geo only works on your WiFi connection at home but not while on a mobile network. In this case please go to the c:geo settings Menu → Settings → Services and enable “Identify as Android Browser”. Afterwards you might have to restart your phone to be sure these settings take effect.
There have been reports, that in rare cases the login still fails although the login data is OK and also “Identify as Android browser” did not help. In this case we recommend resetting your login by long press on “Authorize c:geo” in the platform settings below Menu → Settings → Services. Afterwards you should restart your device and enter the login data again as described above.
As last possibility you can reset c:geo to factory defaults as described here.
If the problem still persists, please contact our support and provide a system log.
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C'est en anglais mais j'espère que cela pourra aider.
Personnellement, j'ai eu un problème identique et il a fallu que je reparamètre mon identifiant et mot de passe en étant connecté en WiFi car en téléphonie GSM (même 4G) cela ne voulait rien faire.
Cette solution est préconisée dans le texte en anglais mais il y en a d'autres.