Explanation
The most likely cause of the issue is that the server has not been joined to the domain. A domain is a logical group of computers and devices that share a common directory service and security policy. A domain controller is a server that manages the domain and authenticates users and computers that want to access domain resources. To log on with domain credentials, a server must be joined to the domain and registered in the directory service. If a server has not been joined to the domain, it will not be recognized or authorized by the domain controller.
References: CompTIA Server+ SK0-005 Certification Study Guide, Chapter 4, Lesson 4.3, Objective 4.3

Explanation
User activity reports were used to identify the security breach in the human resources department. User activity reports are records of the actions and events performed by users on a system or network, such as login/logout times, files accessed or modified, commands executed, or websites visited. User activity reports can help monitor and audit user behavior, detect and investigate security incidents, and enforce policies and compliance. User activity reports can be generated by various tools, such as log management software, security information and event management (SIEM) systems, or user and entity behavior analytics (UEBA) solutions. References: [CompTIA Server+ Certification Exam Objectives], Domain 5.0: Security, Objective
5.2: Given a scenario, apply logical access control methods.

The most likely issue that prevents the newly added server from seeing any of the LUNs on the SAN is that a single-mode fiber cable is used in place of multimode. A single-mode fiber cable is a type of optical fiber cable that has a small core diameter and allows only one mode of light to propagate through it. A single-mode fiber cable can transmit data over long distances at high speeds, but it requires more expensive transceivers and connectors than multimode fiber cables. A multimode fiber cable is a type of optical fiber cable that has a larger core diameter and allows multiple modes of light to propagate through it. A multimode fiber cable can transmit data over short distances at lower speeds than single-mode fiber cables, but it is more compatible and cost-effective than single-mode fiber cables. If a single-mode fiber cable is used in place of multimode, it can cause signal loss, attenuation, or mismatch between the devices. Reference: [CompTIA Server+ Certification Exam Objectives], Domain 3.0: Storage, Objective 3.2: Given a scenario, compare and contrast various storage technologies.

Full Slow backup, fast restore.
Full+Incremental Fast backup, slow restore (compared to Full+Differential).
Full+Differential Slow backup, fast restore (compared to Full+Incremental).