I have created a Module and redeclared the layout for the forgotten password page
//File app/code/Vendor/Module/view/adminhtml/layout/adminhtml_auth_resetpassword.xml
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="login.content">
<block class="Magento\Backend\Block\Template" name="contentt" template="Vendor_Module::admin/resetforgottenpassword.phtml" />
</referenceContainer>
</body>
</page>
The new block just need to have a different name than "content".
//File app/code/Vendor/Module/view/adminhtml/templates/admin/resetforgottenpassword.phtml
<form method="post" data-mage-init='{"form": {}, "validation": {}}' action="<?= /* @escapeNotVerified */ $block->getUrl('*/auth/resetpasswordpost', ['_query' => ['id' => $block->getRequest()->getParam("id"), 'token' => $block->getRequest()->getParam("token")]]) ?>" id="reset-password-form" autocomplete="off"> <fieldset class="admin__fieldset">
<legend class="admin__legend"><span><?= /* @escapeNotVerified */ __('Reset a Password') ?></span></legend><br />
<input name="form_key" type="hidden" value="<?= /* @escapeNotVerified */ $block->getFormKey() ?>" />
<div class="admin__field _required field-password">
<label class="admin__field-label" for="password"><span><?= /* @escapeNotVerified */ __('New Password') ?></span></label>
<div class="admin__field-control">
<input type="password" class="admin__control-text" data-validate="{required:true, 'validate-admin-password':true}" name="password" id="password" placeholder="new password" autocomplete="off" />
</div>
</div>
<div class="admin__field _required field-confirmation">
<label class="admin__field-label" for="confirmation"><span><?= /* @escapeNotVerified */ __('Confirm New Password') ?></span></label>
<div class="admin__field-control">
<input type="password" class="admin__control-text" data-validate="{required:true, 'validate-cpassword':true}" name="confirmation" id="confirmation" placeholder="confirm new password" autocomplete="off" />
</div>
</div>
<div class="form-actions">
<div class="actions">
<button type="submit" title="<?= /* @escapeNotVerified */ __('Reset Password') ?>" class="action-reset action-primary"><span><?= /* @escapeNotVerified */ __('Reset Password') ?></span></button>
</div>
<div class="links">
<a class="action-back" href="<?= /* @escapeNotVerified */ $block->getUrl('adminhtml', ['_nosecret' => true]) ?>"><?= /* @escapeNotVerified */ __('Back to Sign in') ?></a>
</div>
</div>
</fieldset>
</form>
In the original template file the funtions "getResetPasswordLinkToken" and "getUserId" does not exist in the block.
I hope this helps you