It’s likely that the FooEvents Check-ins apps are being temporarily blocked from connecting to your website. This might be due to a security plugin or your website hosting company blocking access.
If you have a security related plugin installed on your website such as Wordfence, please temporarily disable it and try sign-in to the app again. If you are able to sign-in, then the plugin was blocking access and will need to be permanently disabled or configured differently.
You can also ask your hosting company to ensure that there are no security restrictions on the ‘xmlrpc.php’ file which is part of WordPress and that they whitelist all IP addresses on your server for the duration of the event.
Finally, we strongly recommend that you test the app as thoroughly as possible before the event, and to be safe, make sure that you have a contingency plan in place such as a spreadsheet containing all the event attendees for example.