I faced with XML-20108: (Fatal Error) message during 11.2.0.4 installation on RHEL Linux X86-64 platform. Installation screen hangs and installation log has below error messages:

INFO: Stderr Read: org.xml.sax.SAXParseException: <Line 1, Column 1>: XML-20108: (Fatal Error) Start of root element expected.
INFO: Stderr Read: at oracle.xml.parser.v2.XMLError.flushErrorHandler(XMLError.java:422)
INFO: Stderr Read: at oracle.xml.parser.v2.XMLError.flushErrors1(XMLError.java:287)
INFO: Stderr Read: at oracle.xml.parser.v2.NonValidatingParser.parseRootElement(NonValidatingParser.java:461)
INFO: Stderr Read: at oracle.xml.parser.v2.NonValidatingParser.parseDocument(NonValidatingParser.java:402)
INFO: Stderr Read: at oracle.xml.parser.v2.XMLParser.parse(XMLParser.java:244)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiOracleHomeInfoXMLReader.updateHomeProperties(OiiiOracleHomeInfoXMLReader.java:86)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallInventory.updateOracleHomesProperties(OiiiInstallInventory.java:796)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallInventory.updateOracleHomesProperties(OiiiInstallInventory.java:779)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallInventory.readHomes(OiiiInstallInventory.java:735)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallAreaControl.loadPartialInstallInv(OiiiInstallAreaControl.java:782)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallAreaControl.initInstallInv(OiiiInstallAreaControl.java:827)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallAreaControl.loadInstallInventory(OiiiInstallAreaControl.java:598)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallAreaControl.initAreaControl(OiiiInstallAreaControl.java:1991)
INFO: Stderr Read: at oracle.sysman.oii.oiii.OiiiInstallAreaControl.initAreaControl(OiiiInstallAreaControl.java:1944)

This server is already serve 3 different databases versions. Those ares:
– Oracle 9.2.0.8
– Oracle 10.2.0.5
– Oracle 11.1.0.0

After investigate issue, I canceled installation and perform below steps and start instllation again wihtout any errors.

– Rename the existing oraInventory folder.
– Execute the installer.

Referance:
ATS upgrade installer on linux error : Start of root element expected (Doc ID 1988220.1)