VMware on Tuesday reported a major flaw in its latest hypervisor update that prevented customers from turning on virtual machines, which run business applications in virtualized environments within data centers.

The problem, which appeared to be widespread, started when the clocks in host systems crossed into Tuesday. The time change caused the product license for the ESX/ESXi 3.5 Update 2 to expire, preventing customers from turning on virtual machines or moving the VMs.

"VMware is alerting customers and partners of this problem," the company said in an e-mailed statement. "VMware has made this a high-priority issue and has been working around the clock to resolve the problem."

The flaw is within the code of the latest hypervisor update, which was released July 27. VMware, a subsidiary of EMC, said it expects to issue an "express patch" Tuesday to customers in production with the update. A permanent patch is expected by noon Wednesday, once testing is completed. VMware plans to post a page on its site to download the patch.

VMware has not said how many customers are affected, but the problem appears to be widespread, given that the company's discussion forum on the topic recorded nearly 18,000 views as of noon Pacific time. As a quick fix, customers could set back their system clocks to Aug. 10. However, some reported they were legally prevented from changing the clock, which would alter the timestamp on operations.

VMware customers reported that the company's tech support was swamped with calls. "Just got off the phone with tech support and they've been inundated by calls about this very problem," one customer said on the forum.

Others expressed frustration in trying to figure out what was wrong before the problem was disclosed. "Been bashing my head on this last couple of hours," a user wrote.

On July 28, VMware started offering its ESXi hypervisor at no charge, in a move meant to counter Microsoft's Hyper-V, which the software maker released as a $28 feature in Windows Server 2008. The ESXi version represents a version of ESX that's easier to install and use.