The logs in Xcode Server (Server 3) by default point to /Library/Server/XcodeLogs/credserver.log. This takes all of the output from xcscredd and xcscredhandler. If you’re doing a lot of debugging then logs can be pointed to another location, such as another drive. The path to the logs is defined in the /Applications/Server.app/Contents/ServerRoot/System/Library/LogConfiguration directory. The file to edit is a standard property list, XCSCredentialServer.plist.
Once open, look for a key called logPath. Change that to the desired path, such as /Volumes/MyDrive/Logs/credserver.log and then restart the service:
serveradmin stop xcode; serveradmin start xcode