Make sure that the URL of your website where Tickera is installed is in the right form: http://www.your-domain-name.com or https://www.your-domain-name.com. Important thing to know is that http:// or https:// prefix is MANDATORY since it may lead an app to crash if the URL is entered without it.
If you are uncertain of the URL you should enter, you can find the exact URL in Tickera Settings -> API Access tab. More about this can be found here.
Also, issues with the login may be caused by Maintenance plugins. It is not advised to use such plugins at the same time you are using Check-in Apps.