As it has been commented in several forums from version 3.5.3, and therefore the new version 3.6 in testing at present, uses a new method of authentication based on a token ("Login token")
This new method of authentication requires themes that use their own login box to modify the code. Or sites using SSO with any protocol (SAML, CAS, ...) will block the access and the user will not be able to log in to moodle.
We are working on a new version that will solve this problem, as well as new improvements and bug fixes.
It is planned to publish the new version of Adaptable on week 47. If you have updated your site to the latest version of moodle 3.5.3 and have this problem, there is a temporary solution by adding a line in config.php
Other solution is change the login method to "Button". Then the user will be redirected to the moodle login that already include the token .
For more information see the document https://docs.moodle.org/dev/Login_token where the new method is explained, how to apply it and use it in a custom theme and how to avoid problems if you can't access your moodle
(See also https://moodle.org/mod/forum/discuss.php?d=378532#p1526344)
