We have seen several installs of Lync 2013 receive the following error during Step#3: Prepare Current Forest of the “Prepare Active Directory for Lync Server” wizard:
Prepare Forest Active Directory settings execution failed on an unrecoverable error. ForestPrepareTask execution failed on an unrecoverable error.
Reviewing the log shows the following error details:
Error: Existing universal groups were found in “[your AD OU group]”. Specify where to create new Lync Server universal groups explicitly at the command line with the GroupDomain parameter
This occurs during migration of Lync from an earlier version to Lync 2013. The Lync wizard requires any existing RTC* and CS* Security Groups be within the Users OU in Active Directory. Many larger organizations tend to organize such groups into custom OUs. This breaks the Lync 2013 Prepare Active Directory for Lync Server wizard.
- Move all existing RTC* and CS* security groups to the users OU
(See image below)
- Complete the Lync 2013 installation
- Move the RTC* and CS* security groups back to their original location