I have same problem, but clear cache is not solution. Last time i solved after restore afected files from backup, but after upgrade to openmediavault 4.1.1-1 not work.
Code
Error #0: OMV\AssertException: The property 'nullpasswords' does not exist in the model 'conf.service.smb'. in /usr/share/php/openmediavault/config/configobject.inc:71 Stack trace: #0 /usr/share/php/openmediavault/config/configobject.inc(186): OMV\Config\ConfigObject->assertExists('nullpasswords') #1 /usr/share/php/openmediavault/config/configobject.inc(271): OMV\Config\ConfigObject->set('nullpasswords', '1', false) #2 /usr/share/php/openmediavault/config/configobject.inc(233): OMV\Config\ConfigObject->setFlatAssoc(Array, false, false) #3 /usr/share/php/openmediavault/config/database.inc(90): OMV\Config\ConfigObject->setAssoc(Array, false) #4 /usr/share/openmediavault/engined/module/samba.inc(72): OMV\Config\Database->get('conf.service.sm...') #5 /usr/share/openmediavault/engined/rpc/services.inc(56): OMVModuleSamba->getStatus() #6 [internal function]: OMVRpcServiceServices->getStatus(Array, Array) #7 /usr/share/php/openmediavault/rpc/serviceabstract.inc(123): call_user_func_array(Array, Array) #8 /usr/share/php/openmediavault/rpc/rpc.inc(86): OMV\Rpc\ServiceAbstract->callMethod('getStatus', Array, Array) #9 /usr/sbin/omv-engined(536): OMV\Rpc\Rpc::call('Services', 'getStatus', Array, Array, 1) #10 {main}