Was installing a Skype for Business server the other day, and the simple task of preparing the forest failed. I am always on the alert when doing Active Directory forest wide tasks as prepare schema and prepare forest, so it is not fun to see error messages during these tasks.
This time there was nothing but lots of scary errors.
I know this domain has several trusts configured, so it looks like the wizard get confused of where to put these groups. Next step was to run prepare forest from PowerShell so that I was able to provide all this information to the command.
Enable-CsAdForest -GroupDomain s4b.local -GroupDomainController s4b-dc1.s4b.local -GlobalCatalog s4b-dc1.s4b.local
And finally success. The command completed without warnings and errors.