The fact that the issue is occurring with different users each day suggests that the problem is not with the individual PCs or their configurations. Additionally, rebooting the PCs did not resolve the issue, which further supports this idea.
The symptoms described could be caused by an overloaded CAM (Content Addressable Memory) table in the switch. The CAM table is used to map MAC addresses to switch ports, allowing the switch to forward frames to the correct destination. If the table becomes full, new MAC addresses cannot be learned, and frames may be dropped, resulting in connectivity issues.
Incorrect firewall settings, inappropriate VLAN assignment, hardware failure, and DHCP scope exhaustion are also possible causes of network connectivity issues, but they are less likely to be the cause of the described symptoms in this particular scenario.