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The error is telling you that there is a conflict with login credentials. I think you should declare thymeleaf in the index.html . As follows:
xmlns:sec="http://www.thymeleaf.org/thymeleaf- extras-springsecurity3" xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout">
<meta charset="utf-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />