I had the same error. After deleting the file c:\Users\<Username>\soapui-settings.xml and restart soapUI it works fine.
On this PC we've used soapUI beginning with version 2. So I guess every update has changed the setting-file. Maybe one (old) setting has produced this error. Here the content of the file:
<?xml version="1.0" encoding="UTF-8"?>
<con:soapui-settings xmlns:con="
http://eviware.com/soapui/config"><con:setting id="WsdlSettings@excluded-types"><con:entry xmlns:con="
http://eviware.com/soapui/config">schema@http://www.w3.org/2001/XMLSchema</con:entry></con:setting><con:setting id="WsdlSettings@name-with-binding">true</con:setting><con:setting id="HttpSettings@http_version">1.1</con:setting><con:setting id="HttpSettings@max_total_connections">2000</con:setting><con:setting id="HttpSettings@response-compression">true</con:setting><con:setting id="HttpSettings@leave_mockengine">true</con:setting><con:setting id="UISettings@auto_save_projects_on_exit">true</con:setting><con:setting id="UISettings@show_descriptions">true</con:setting><con:setting id="WsdlSettings@xml-generation-always-include-optional-elements">true</con:setting><con:setting id="WsaSettings@useDefaultRelatesTo">true</con:setting><con:setting id="WsaSettings@useDefaultRelationshipType">true</con:setting><con:setting id="UISettings@show_startup_page">true</con:setting><con:setting id="UISettings@gc_interval">60</con:setting><con:setting id="WsdlSettings@cache-wsdls">true</con:setting><con:setting id="WsdlSettings@pretty-print-response-xml">true</con:setting><con:setting id="HttpSettings@include_request_in_time_taken">true</con:setting><con:setting id="HttpSettings@include_response_in_time_taken">true</con:setting><con:setting id="UISettings@auto_save_interval">0</con:setting><con:setting id="WsaSettings@soapActionOverridesWsaAction">true</con:setting><con:setting id="WsaSettings@overrideExistingHeaders">true</con:setting><con:setting id="WsaSettings@enableForOptional">true</con:setting><con:setting id="WSISettings@location">C:\_Apps\soapui-3.5\bin\..\wsi-test-tools</con:setting><con:setting id="GlobalPropertySettings@properties"><xml-fragment/></con:setting><con:setting id="RecentProjects"><entry key="" value="CFD" xmlns="
http://eviware.com/soapui/config"/></con:setting><con:setting id="HttpSettings@user-agent"/><con:setting id="HttpSettings@request-compression">None</con:setting><con:setting id="HttpSettings@disable_response_decompression">false</con:setting><con:setting id="HttpSettings@close-connections">true</con:setting><con:setting id="HttpSettings@chunking_threshold"/><con:setting id="HttpSettings@authenticate-preemptively">false</con:setting><con:setting id="HttpSettings@expect-continue">false</con:setting><con:setting id="HttpSettings@encoded_urls">false</con:setting><con:setting id="HttpSettings@bind_address"/><con:setting id="HttpSettings@socket_timeout"/><con:setting id="HttpSettings@max_response_size"/><con:setting id="HttpSettings@max_connections_per_host"/><con:setting id="HttpSettings@enable_mock_wire_log">false</con:setting><con:setting id="ProxySettings@host"/><con:setting id="ProxySettings@port"/><con:setting id="ProxySettings@username"/><con:setting id="ProxySettings@password"/><con:setting id="ProxySettings@excludes"/><con:setting id="ProxySettings@enableProxy">false</con:setting><con:setting id="SSLSettings@keyStore"/><con:setting id="SSLSettings@keyStorePassword"/><con:setting id="SSLSettings@enableMockSSL">false</con:setting><con:setting id="SSLSettings@mockPort"/><con:setting id="SSLSettings@mockKeyStore"/><con:setting id="SSLSettings@mockPassword"/><con:setting id="SSLSettings@mockKeyStorePassword"/><con:setting id="SSLSettings@mockTrustStore"/><con:setting id="SSLSettings@mockTrustStorePassword"/><con:setting id="SSLSettings@needClientAuthentication">false</con:setting><con:setting id="WsdlSettings@xml-generation-type-example-value">false</con:setting><con:setting id="WsdlSettings@xml-generation-type-comment-type">false</con:setting><con:setting id="WsdlSettings@attachment-parts">false</con:setting><con:setting id="WsdlSettings@allow-incorrect-contenttype">false</con:setting><con:setting id="WsdlSettings@schema-directory"/><con:setting id="WsdlSettings@strict-schema-types">false</con:setting><con:setting id="WsdlSettings@compression-limit"/><con:setting id="WsdlSettings@pretty-print-project-files">false</con:setting><con:setting id="UISettings@close-projects">false</con:setting><con:setting id="UISettings@order-projects">false</con:setting><con:setting id="UISettings@order-requests">false</con:setting><con:setting id="UISettings@create_backup">false</con:setting><con:setting id="UISettings@backup_folder"/><con:setting id="UISettings@normalize_line-breaks">false</con:setting><con:setting id="UISettings@desktop-type">Default</con:setting><con:setting id="UISettings@native-laf">false</con:setting><con:setting id="UISettings@dont-disable-groovy-log">false</con:setting><con:setting id="UISettings@show_logs_at_startup">false</con:setting><con:setting id="UISettings@raw_response_message_size_show">10000</con:setting><con:setting id="UISettings@raw_request_message_size_show">10000</con:setting><con:setting id="UISettings@editor-font">Monospaced.plain 11</con:setting><con:setting id="UISettings@no_resize_request_editor">false</con:setting><con:setting id="UISettings@start_with_request_tabs">false</con:setting><con:setting id="UISettings@auto_validate_request">false</con:setting><con:setting id="UISettings@abort_on_invalid_request">false</con:setting><con:setting id="UISettings@auto_validate_response">false</con:setting><con:setting id="UISettings@show_xml_line_numbers">false</con:setting><con:setting id="UISettings@show_groovy_line_numbers">false</con:setting><con:setting id="ToolsSettings@jbossws_wstools"/><con:setting id="ToolsSettings@axis_1_X"/><con:setting id="ToolsSettings@axis_2"/><con:setting id="ToolsSettings@jwsdp_wscompile"/><con:setting id="ToolsSettings@jwsdp_wsimport"/><con:setting id="ToolsSettings@javac"/><con:setting id="ToolsSettings@dotnet_wsdl"/><con:setting id="ToolsSettings@cxf"/><con:setting id="ToolsSettings@xfire"/><con:setting id="ToolsSettings@gsoap"/><con:setting id="ToolsSettings@ant"/><con:setting id="ToolsSettings@xmlbeans"/><con:setting id="ToolsSettings@jaxb"/><con:setting id="ToolsSettings@tcpmon"/><con:setting id="ToolsSettings@wsa"/><con:setting id="ToolsSettings@wadl2java"/><con:setting id="ToolsSettings@hermesjms"/><con:setting id="WSISettings@verbose">false</con:setting><con:setting id="WSISettings@results_type">all</con:setting><con:setting id="WSISettings@messageEntry">false</con:setting><con:setting id="WSISettings@failureMessage">false</con:setting><con:setting id="WSISettings@assertionDescription">false</con:setting><con:setting id="WSISettings@showLog">false</con:setting><con:setting id="WSISettings@outputFolder"/><con:setting id="GlobalPropertySettings@enableOverride">false</con:setting><con:setting id="SecuritySettings@shadowProxyPassword"/><con:setting id="WebRecordingSettings@excluded-headers"><![CDATA[<xml-fragment><con:entry xmlns:con="
http://eviware.com/soapui/config">Cookie</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Set-Cookie</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Referer</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Keep-Alive</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Connection</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Proxy-Connection</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Pragma</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Cache-Control</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Transfer-Encoding</con:entry><con:entry xmlns:con="
http://eviware.com/soapui/config">Date</con:entry></xml-fragment>]]></con:setting></con:soapui-settings>