The esx.problem.vmfs.extent.offline message is received when an ESXi host loses connection to a storage device that backs an entire VMFS datastore or any of its extents.
Impact
I/O to virtual disk (VMDK) files that reside on the offline device might slow down or fail. This might cause virtual machines that have VMDKs residing on that device to become unresponsive or even fail.
Example
This loss of connection can happen when a switch or cable that connects the device to the ESXi host is disconnected or when the device is reformatted to be used by another volume.