How can I resolve this problem ..

Odoo ServerError
Traceback (most recent call last)

File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 648, in _handle_exception    return super(JsonRequest, self)._handle_exception(exception)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 685, in dispatch    result = self._call_function(**self.params)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 321, in _call_function    return checked_call(self.db, *args, **kwargs)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/service/model.py", line 118, in wrapper    return f(dbname, *args, **kwargs)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 314, in checked_call    result = self.endpoint(*a, **kw)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 964, in __call__    return self.method(*args, **kw)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/http.py", line 514, in response_wrap    response = f(*args, **kw)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/addons/web/controllers/main.py", line 892, in call_button    action = self._call_kw(model, method, args, {})  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/addons/web/controllers/main.py", line 880, in _call_kw    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/api.py", line 250, in wrapper    return old_api(self, *args, **kwargs)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/addons/base/module/module.py", line 459, in button_immediate_install    return self._button_immediate_function(cr, uid, ids, self.button_install, context=context)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/api.py", line 250, in wrapper    return old_api(self, *args, **kwargs)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/addons/base/module/module.py", line 534, in _button_immediate_function    registry = openerp.modules.registry.RegistryManager.new(cr.dbname, update_module=True)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/modules/registry.py", line 386, in new    openerp.modules.load_modules(registry._db, force_demo, status, update_module)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/modules/loading.py", line 340, in load_modules    loaded_modules, update_module)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/modules/loading.py", line 239, in load_marked_modules    loaded, processed = load_module_graph(cr, graph, progressdict, report=report, skip_modules=loaded_modules, perform_checks=perform_checks)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/modules/loading.py", line 156, in load_module_graph    _load_data(cr, module_name, idref, mode, kind='data')  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/modules/loading.py", line 98, in _load_data    tools.convert_file(cr, module_name, filename, idref, mode, noupdate, kind, report)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/tools/convert.py", line 849, in convert_file    convert_xml_import(cr, module, fp, idref, mode, noupdate, report)  
File "/home/nebil/Work/Eclipse/workspace/odoo2/openerp/tools/convert.py", line 915, in convert_xml_import    doc = etree.parse(xmlfile)  
File "src/lxml/lxml.etree.pyx", line 3427, in lxml.etree.parse (src/lxml/lxml.etree.c:79841)  
File "src/lxml/parser.pxi", line 1814, in lxml.etree._parseDocument (src/lxml/lxml.etree.c:116443)  
File "src/lxml/parser.pxi", line 1834, in lxml.etree._parseFilelikeDocument (src/lxml/lxml.etree.c:116735)  
File "src/lxml/parser.pxi", line 1729, in lxml.etree._parseDocFromFilelike (src/lxml/lxml.etree.c:115496)  
File "src/lxml/parser.pxi", line 1150, in lxml.etree._BaseParser._parseDocFromFilelike (src/lxml/lxml.etree.c:110370)  
File "src/lxml/parser.pxi", line 584, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:103584)  
File "src/lxml/parser.pxi", line 694, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:105238)  
File "src/lxml/parser.pxi", line 624, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:104147)

XMLSyntaxError: xmlParseEntityRef: no name, line 257, column 55