Hi Gerben Verbrugghe,
I assume that you are working with the 10.2.x release of Explorer. This version of the application does not support public accounts. If you have access to a paid subscription account you can use that account info instead.
If you are looking to use your public account with the Explorer application, I would consider enrolling in our beta program here:
https://community.esri.com/message/691966-android-explorer-beta-release-signup-and-install-informati...
The beta program features the new version of explorer coming in the next few months. This newer version of Explorer removes the requirement of having an organizational account so you should be able to login with the credentials that you were using. If you do not wish to enroll in the beta, this version should be available to you within the next few months.
I hope this helps!
