Look for routing problems on the LAN connections physically connected to that Cisco. Including things like VLAN settings on the switches. Look for DHCP problems, DNS problems, etc.

If the user's VPN credentials are being authenticated via some kind of centralized system, for example, if users are authenticating with the VPN via their corporate Active Directory accounts, then look for problems between the Cisco and the domain controller and the path to authenticate with it.

Find out the timing of the first reports of connection failures. Then find out what kinds of changes were made (if any) right before that. For example, someone replaced a switch or a Wifi router, or made a change to the DNS settings, two days before the VPN problems started happening.

Check for a possible breach or malware. Or perhaps an update to security software which might be interfering.