SoapUI 5.6.0 won't start

Problem: I can't run SoapUI 5.6.0 on Linux. I see only splash screen, sometimes even the splash screen won't show up.I am using Ubuntu 20.04.1 LTS. It's newly installed.

In the error.log is this exception:

Exception in thread "AWT-EventQueue-0" java.lang.NoSuchMethodError: 'boolean org.apache.xmlbeans.XmlOptionsBean.isLoadDTDGrammar()'
	at org.apache.xmlbeans.impl.common.SAXHelper.saxFactory(SAXHelper.java:70)
	at org.apache.xmlbeans.impl.common.SAXHelper.newXMLReader(SAXHelper.java:46)
	at org.apache.xmlbeans.impl.store.Locale.getSaxLoader(Locale.java:3066)
	at org.apache.xmlbeans.impl.store.Locale.parseToXmlObject(Locale.java:1272)
	at org.apache.xmlbeans.impl.store.Locale.parseToXmlObject(Locale.java:1259)
	at org.apache.xmlbeans.impl.schema.SchemaTypeLoaderBase.parse(SchemaTypeLoaderBase.java:345)
	at org.apache.xmlbeans.impl.schema.SchemaTypeLoaderBase.parse(SchemaTypeLoaderBase.java:252)
	at com.eviware.soapui.config.SoapuiSettingsDocumentConfig$Factory.parse(SoapuiSettingsDocumentConfig.java:59)
	at com.eviware.soapui.DefaultSoapUICore.initSettings(DefaultSoapUICore.java:259)
	at com.eviware.soapui.SwingSoapUICore.initSettings(SwingSoapUICore.java:99)
	at com.eviware.soapui.DefaultSoapUICore.init(DefaultSoapUICore.java:141)
	at com.eviware.soapui.StandaloneSoapUICore.<init>(StandaloneSoapUICore.java:38)
	at com.eviware.soapui.SoapUI$SoapUIRunner.run(SoapUI.java:791)
	at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
	at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
	at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
	at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

I tried:

- run an instance that I had on the previous OS (disk backup).

- install new SoapUI 5.6.0 with installator that I newly downloaded.

- remove all folders and setting files (default-soapui-workspace.xml, soapui-settings.xml, etc).

- use previous backuped folders and setting files from an old OS instance. 

- use different versions of java (8 oracle ,11 openjdk ,13 oracle)

- replace lib/xmlbeans-3.1.1-sb-fixed.jar with xmlbeans-3.1.0.jar

nothing works