I had the same problem. This turned up in my logs:
> [no app in context] Error: Icewind\SMB\Exception\ForbiddenException: Invalid request for /<username> (ForbiddenException) at <<closure>>
>
> 0. /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Native/NativeState.php line 62
> Icewind\SMB\Exception\Exception::fromMap({1: "Icewind\\SM ... "}, 1, "/<username>")
> 1. /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Native/NativeState.php line 74
> Icewind\SMB\Native\NativeState->handleError("/<username>")
> 2. /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Native/NativeState.php line 184
> Icewind\SMB\Native\NativeState->testResult("*** sensitive parameter replaced ***", "smb://<IP-ADRES-SMBSERVER>/usershares/<username>")
> 3. /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Native/NativeShare.php line 133
> Icewind\SMB\Native\NativeState->stat("smb://<IP-ADDRESS SMBSERVER>/usershares/<username>")
> 4. /var/www/nextcloud/apps/files_external/3rdparty/icewind/smb/src/Native/NativeShare.php line 112
> Icewind\SMB\Native\NativeShare->getStat("/<username>")
> 5. /var/www/nextcloud/apps/files_external/lib/Lib/Storage/SMB.php line 178
> Icewind\SMB\Native\NativeShare->stat("/<username>")
> 6. /var/www/nextcloud/apps/files_external/lib/Lib/Storage/SMB.php line 296
> OCA\Files_External\Lib\Storage\SMB->getFileInfo("/<username>")
> 7. /var/www/nextcloud/lib/private/Files/Storage/Common.php line 454
> OCA\Files_External\Lib\Storage\SMB->stat("")
> 8. /var/www/nextcloud/apps/files_external/lib/Lib/Storage/SMB.php line 612
> OC\Files\Storage\Common->test()
> 9. /var/www/nextcloud/apps/files_external/lib/config.php line 274
> OCA\Files_External\Lib\Storage\SMB->test("*** sensitive parameter replaced ***", "*** sensitive parameter replaced ***")
> 10. /var/www/nextcloud/apps/files_external/lib/Controller/StoragesController.php line 256
> OC_Mount_Config::getBackendStatus("*** sensitive parameters replaced ***")
> 11. /var/www/nextcloud/apps/files_external/lib/Controller/UserGlobalStoragesController.php line 122
> OCA\Files_External\Controller\StoragesController->updateStorageStatus("*** sensitive parameters replaced ***")
> 12. /var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 170
> OCA\Files_External\Controller\UserGlobalStoragesController->show(2, "*** sensitive parameter replaced ***")
> 13. /var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 99
> OC\AppFramework\Http\Dispatcher->executeController(OCA\Files_Extern ... {}, "show")
> 14. /var/www/nextcloud/lib/private/AppFramework/App.php line 126
> OC\AppFramework\Http\Dispatcher->dispatch(OCA\Files_Extern ... {}, "show")
> 15. /var/www/nextcloud/lib/private/AppFramework/Routing/RouteActionHandler.php line 47
> OC\AppFramework\App::main("OCA\\Files_Exte ... r", "show", OC\AppFramework\ ... {}, {id: "2",_route: ... "})
> 16. <<closure>>
> OC\AppFramework\Routing\RouteActionHandler->__invoke({id: "2",_route: ... "})
> 17. /var/www/nextcloud/lib/private/Route/Router.php line 297
> undefinedundefinedcall_user_func(OC\AppFramework\ ... {}, {id: "2",_route: ... "})
> 18. /var/www/nextcloud/lib/base.php line 997
> OC\Route\Router->match("/apps/files_ext ... 2")
> 19. /var/www/nextcloud/index.php line 42
> OC::handleRequest()
>
> GET /index.php/apps/files_external/userglobalstorages/2?testOnly=false
> from <IP ADDRESS gateway> by <username> at 2020-01-24T07:50:22+00:00
The problem occurred after I changed my Active Directory password (I have LDAP integration). I use an external storage SMB for which I use my AD credentials. After I gave in my new credentials in Nextcloud (in the browser), the error in the sync client was gone.
Hopefully this can help others.
kr
Wouter