Experiencing the same problem in Buster with a vanilla installation, but probably with a different cause.
Using jabref 3.8.2+ds-12, jabref is unable to connect to libreoffice automatically or manually. I see this output from jabref when I attempt to connect: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap') at net.sf.jabref.gui.openoffice.OpenOfficePanel.addURL(OpenOfficePanel.java:484) ~[JabRef-3.8.2.jar:?] at net.sf.jabref.gui.openoffice.OpenOfficePanel.connect(OpenOfficePanel.java:419) ~[JabRef-3.8.2.jar:?] at net.sf.jabref.gui.openoffice.OpenOfficePanel.lambda$initPanel$0(OpenOfficePanel.java:141) ~[JabRef-3.8.2.jar:?] at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967) ~[?:?] at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308) ~[?:?] at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405) ~[?:?] at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262) ~[?:?] at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279) ~[?:?] at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:297) ~[?:?] at java.awt.Component.processMouseEvent(Component.java:6632) ~[?:?] at javax.swing.JComponent.processMouseEvent(JComponent.java:3342) ~[?:?] at java.awt.Component.processEvent(Component.java:6397) ~[?:?] at java.awt.Container.processEvent(Container.java:2263) ~[?:?] at java.awt.Component.dispatchEventImpl(Component.java:5008) ~[?:?] at java.awt.Container.dispatchEventImpl(Container.java:2321) ~[?:?] at java.awt.Component.dispatchEvent(Component.java:4840) ~[?:?] at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918) ~[?:?] at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547) ~[?:?] at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488) ~[?:?] at java.awt.Container.dispatchEventImpl(Container.java:2307) ~[?:?] at java.awt.Window.dispatchEventImpl(Window.java:2772) ~[?:?] at java.awt.Component.dispatchEvent(Component.java:4840) ~[?:?] at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772) ~[?:?] at java.awt.EventQueue$4.run(EventQueue.java:721) ~[?:?] at java.awt.EventQueue$4.run(EventQueue.java:715) ~[?:?] at java.security.AccessController.doPrivileged(Native Method) ~[?:?] at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) ~[?:?] at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95) ~[?:?] at java.awt.EventQueue$5.run(EventQueue.java:745) ~[?:?] at java.awt.EventQueue$5.run(EventQueue.java:743) ~[?:?] at java.security.AccessController.doPrivileged(Native Method) ~[?:?] at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) ~[?:?] at java.awt.EventQueue.dispatchEvent(EventQueue.java:742) ~[?:?] at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203) [?:?] at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124) [?:?] at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113) [?:?] at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109) [?:?] at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) [?:?] at java.awt.EventDispatchThread.run(EventDispatchThread.java:90) [?:?] JabRef was able to connect to LibreOffice fine for me in Stretch. Thanks, Gary p.s. I inadvertently sent the last email as HTML (sorry), so I am resending it as plain text.