After deactivating other plugins I get the following in the error-log:
[26-Aug-2016 17:05:12 UTC] PHP Warning: http_build_query(): Parameter 1 expected to be Array or Object. Incorrect value given in /...../wp-includes/Requests/Transport/cURL.php on line 507
[26-Aug-2016 17:05:14 UTC] Created calendar: Created calendar. Please reload this page to try again. If this error persists, please contact us at labs@time.ly. In your report please include the information below.
API URL: https://api.time.ly/api/calendars.
Detail: 401 - Unauthorized. - raw error: Array
Hope this will give a clue what's going wrong? I did also send an e-mail as proposed above.