Further to my original post I've tried the following with no success:
1) Opening local desktop client via Firefox as http://localhost:7633/desktop/login.jsp. This gives exactly the same problem.
2) Complete system re-boot does not change the problem.
Attached are the log files from today since the complete system re-boot. Note that the "LDAP: error code 17 - zimbraPrefMailFlashIcon: attribute type undefined]" error seems to have started in the log files only after upgrading to Beta5 and then adjusting some preferences.
There is also a lot of warning messages in the Error Console for Prism. (How do I get a copy of the information logged in the console to post here?). There are references to undefined variable, undefined properties etc. from several files. Here are some examples cut and pasted from hundreds in the error console:
Warning: reference to undefined property this._buttons[id]
Source File: http://localhost:7633/zimbra/js/zimbraMail/share/view/ZmAppChooser.js?v=081205141935
Warning: reference to undefined property AjxDebug.DBG1
Source File: http://localhost:7633/zimbra/js/zimbraMail/core/ZmRequestMgr.js?v=081205141935
Warning: reference to undefined property AjxDebug.DBG2
Source File: http://localhost:7633/zimbra/js/zimbraMail/core/ZmZimbraMail.js?v=081205141935
Warning: assignment to undeclared variable ZmZimletTreeController
Source File: http://localhost:7633/zimbra/js/ajax/boot/AjxPackage.js?v=081205141935
Warning: reference to undefined property acct.trees[ZmOrganizer.TREE_TYPE[type]]
Source File: http://localhost:7633/zimbra/js/zimbraMail/core/ZmAppCtxt.js?v=081205141935
Warning: assignment to undeclared variable com_zimbra_local
Source File: http://localhost:7633/service/zimlet/res/Zimlets-nodev_all.js?language=en
Warning: reference to undefined property this.value[key]
Source File: http://localhost:7633/zimbra/js/zimbraMail/share/model/ZmSetting.js?v=081205141935
Warning: reference to undefined property AjxDebug.DBG3
Source File: http://localhost:7633/zimbra/js/zimbraMail/core/ZmAppViewMgr.js?v=081205141935