Adprep was unable to update forest information. Status/Consequence Adprep requires access to existing forest-wide information ...

Adprep was unable to update forest information. 
[Status/Consequence]
Adprep requires access to existing forest-wide information from the schema master in order to complete this operation.
[User Action]
Check the log file, ADPrep.log, in the %1 directory for more information.