This release of VMware vCenter Server 5.1 Update 1b delivers a number of bug fixes that have been documented in the Resolved Issues section.

Resolved Issues
This release resolves the following issues in the vCenter Server, vSphere Client, and vSphere Web Client subject area:

vCenter Server stops responding frequently with the Win32 exception: Access Violation (0xc0000005) message in the log file
Two concurrent calls to retrieve information from the same host might cause the vCenter Server to stop responding frequently. The vpxd log files might display messages similar to the following:
[16064 verbose ‘VpxProfiler’ opID=e05b1f14] [1-] [VpxdVdb] LoadCLOB: vim.host.SystemResourceInfo (took 1014 ms)
[16064 panic ‘Default’ opID=e05b1f14] (Log recursion level 2) Win32 exception: Access Violation (0xc0000005)
Read (0) at address ffffffffffffffff
rip: 0000000000ad522b rsp: 00000000d4c4dcd0 rbp: 000000002d8f7040
rax: 000000018035334a rbx: 000000000b94e210 rcx: 00000000130da260
rdx: 0000000000000001 rdi: 0000000011633f50 rsi: 000000002c95cbb0
r8: 0000000000000518 r9: 0000000000000518 r10: 00000000034f0080
r11: 00000000000d700c r12: 000000000000002f r13: 000000001621cf08
r14: 0000000177eff330 r15: 000000002c95a228

This issue is resolved in this release.

Exporting all system log files of a host by using vSphere Web Client might cause all virtual machines running on the host to stop responding
When you use the vSphere Web client to export the system log files of a host, the HungVM manifest option is available in the list of system log files that can be exported. If you select HungVM or select all the log files and generate the log bundle, all the virtual machines running on the selected host might stop responding.

This issue is resolved in this release. The HungVM log is no longer available as an option in the list of system logs.

vSphere Web Client might fail to display the advanced performance charts
When you use a German locale vSphere Web Client to view advanced performance charts, an error message similar to the following might be displayed:
Performance data is not currently available for this entry

In addition, a notification message similar to the following might also be displayed:
For input string .

This issue is resolved in this release.

vSphere Web Client might fail to display all the plug-ins if legacy script plug-ins are enabled
When you enable the use of legacy script plug-ins along with other vSphere web client plug-ins by adding the scriptPlugin.enabled=true line in the webclient.properties file, vSphere Web Client might fail to display all the enabled plug-ins. Either legacy script plug-ins or only regular plug-ins are visible in the vSphere Web Client.

This issue is resolved in this release.

ESX Agent Manager might uninstall host agents when settings of DRS affinity rule, advanced options in VMware HA, or power management options for hosts in VMware DPM are removed
When you remove DRS affinity rules from a cluster or similarly change parameters of VMware HA or VMware DPM, an issue with the PropertyCollector service might cause the ESX Agent Manager to delete ESX host agent and associated VIBs from the ESXi hosts.
The Tasks & Events tab for the cluster might display the entry Task: Uninstall Agent from user com.vmware.vim.eam.

This issue is resolved in this release.

All virtual machine folders on a VMFS volume are deleted after a single virtual machine folder is attempted for delete resulting in failure, and you choose Delete again by using the Datastore Browser
When you attempt to delete a virtual machine folder while the virtual machine is powered on, a message similar to Cannot delete file. [VMFS] <virtual_Machine_Name> is displayed. If you click the Delete button again without closing the datastore browser or changing the selection, and attempt to delete the same virtual machine folder that failed to delete earlier, all virtual machine folders on the VMFS volume might now get deleted.

This issue is resolved in this release.

More info at vmware.com