What is the solution for login failures after enabling selective authentication in a forest trust?

Enabling selective authentication in a forest trust allows for a more controlled access scenario, meaning that by default, users from the trusting forest cannot access resources in the trusted forest unless explicitly granted permissions. Therefore, to resolve login failures that occur due to insufficient permissions, one effective solution is to add users to the Discretionary Access Control List (DACL) of the computer accounts in the trusting forest.

Doing this provides the necessary permissions for those specific users or groups, enabling them to authenticate successfully. This solution retains the benefits of selective authentication while ensuring that specific users have the necessary access they require to resources in the trusted forest.

The alternatives offered in the question do not address the immediate issue of login failures in the context of selective authentication. For instance, disabling selective authentication or recreating the trust with full authentication negates the controlled access provided by selective authentication, which may not be desirable for security policies. Increasing permissions of user accounts might lead to broader access than intended without providing the necessary specificity that comes from modifying the DACL. Thus, the first option is the most targeted and effective approach to fix the login issues while maintaining selective authentication.