Try to clear the browser cache and try again. I realized that a problem as I update to NC 16.0.4 and this was the recommended solution for the problem.
Could not solve the problem by clearing the cache.
With NC17 I did a total reinstall. Everything was fine at the start. After I installed 2-Factor-auth with TOTP it started again. I activated log level 0 and that showed the following:
[core] Debug: OC\AppFramework\Middleware\Security\Exceptions\CrossSiteRequestForgeryException: CSRF check failed at <<closure>>
0. /homepages/21/XXX/htdocs/privatecloud/lib/private/AppFramework/Middleware/MiddlewareDispatcher.php line 95
OC\AppFramework\Middleware\Security\SecurityMiddleware->beforeController(OCA\Provisioning ... {}, "getUsersDetails")
1. /homepages/21/XXX/htdocs/privatecloud/lib/private/AppFramework/Http/Dispatcher.php line 97
OC\AppFramework\Middleware\MiddlewareDispatcher->beforeController(OCA\Provisioning ... {}, "getUsersDetails")
2. /homepages/21/XXX/htdocs/privatecloud/lib/private/AppFramework/App.php line 126
OC\AppFramework\Http\Dispatcher->dispatch(OCA\Provisioning ... {}, "getUsersDetails")
3. /homepages/21/XXX/htdocs/privatecloud/lib/private/AppFramework/Routing/RouteActionHandler.php line 47
OC\AppFramework\App::main("OCA\\Provisioni ... r", "getUsersDetails", OC\AppFramework\ ... {}, {_route: "ocs.pr ... "})
4. <<closure>>
OC\AppFramework\Routing\RouteActionHandler->__invoke({_route: "ocs.pr ... "})
5. /homepages/21/XXX/htdocs/privatecloud/lib/private/Route/Router.php line 297
undefinedundefinedcall_user_func(OC\AppFramework\ ... {}, {_route: "ocs.pr ... "})
6. /homepages/21/XXX/htdocs/privatecloud/ocs/v1.php line 82
OC\Route\Router->match("/ocsapp/cloud/users/details")
7. /homepages/21/XXX/htdocs/privatecloud/ocs/v2.php line 24
undefinedundefinedrequire_once("/homepages/21/d ... p")
GET /ocs/v2.php/cloud/users/details?offset=0&limit=25&search=
from XXX at 2019-11-28T13:24:39+00:00