Thanks a lot for your answer!
Today there was an Error in the nextcloud error log. That’s what helped me to find the cause.
The main problem was caused by the Collabora Online - Built-in CODE Server
6.4.14 app. Disabling this made everything fast again. Maybe the error log helps to find a bug.
Error richdocuments GuzzleHttp\Exception\ConnectException: cURL error 28: Connection timed out after 45001 milliseconds (see https://curl.haxx.se/libcurl/c/libcurl-errors.html)
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Handler/CurlFactory.php - line 155:
GuzzleHttp\Handler\CurlFactory::createRejection()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Handler/CurlFactory.php - line 105:
GuzzleHttp\Handler\CurlFactory::finishError()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Handler/CurlHandler.php - line 43:
GuzzleHttp\Handler\CurlFactory::finish()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Handler/Proxy.php - line 28:
GuzzleHttp\Handler\CurlHandler->__invoke()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Handler/Proxy.php - line 51:
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}("*** sensiti ... *")
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/PrepareBodyMiddleware.php - line 37:
GuzzleHttp\Handler\Proxy::GuzzleHttp\Handler\{closure}("*** sensiti ... *")
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Middleware.php - line 29:
GuzzleHttp\PrepareBodyMiddleware->__invoke()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/RedirectMiddleware.php - line 70:
GuzzleHttp\Middleware::GuzzleHttp\{closure}("*** sensiti ... *")
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Middleware.php - line 59:
GuzzleHttp\RedirectMiddleware->__invoke()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/HandlerStack.php - line 71:
GuzzleHttp\Middleware::GuzzleHttp\{closure}("*** sensiti ... *")
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Client.php - line 361:
GuzzleHttp\HandlerStack->__invoke()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Client.php - line 163:
GuzzleHttp\Client->transfer()
/var/www/html/nextcloud/3rdparty/guzzlehttp/guzzle/src/Client.php - line 183:
GuzzleHttp\Client->requestAsync()
/var/www/html/nextcloud/lib/private/Http/Client/Client.php - line 233:
GuzzleHttp\Client->request()
/var/www/html/nextcloud/apps/richdocuments/lib/WOPI/DiscoveryManager.php - line 81:
OC\Http\Client\Client->get()
/var/www/html/nextcloud/apps/richdocuments/lib/WOPI/DiscoveryManager.php - line 56:
OCA\Richdocuments\WOPI\DiscoveryManager->fetchFromRemote()
/var/www/html/nextcloud/apps/richdocuments/lib/WOPI/Parser.php - line 41:
OCA\Richdocuments\WOPI\DiscoveryManager->get()
/var/www/html/nextcloud/apps/richdocuments/lib/TokenManager.php - line 210:
OCA\Richdocuments\WOPI\Parser->getUrlSrc()
/var/www/html/nextcloud/apps/richdocuments/lib/Controller/DocumentController.php - line 256:
OCA\Richdocuments\TokenManager->getToken("*** sensiti ... *")
/var/www/html/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 169:
OCA\Richdocuments\Controller\DocumentController->index()
/var/www/html/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 100:
OC\AppFramework\Http\Dispatcher->executeController()
/var/www/html/nextcloud/lib/private/AppFramework/App.php - line 152:
OC\AppFramework\Http\Dispatcher->dispatch()
/var/www/html/nextcloud/lib/private/Route/Router.php - line 308:
OC\AppFramework\App::main()
/var/www/html/nextcloud/lib/base.php - line 1008:
OC\Route\Router->match()
/var/www/html/nextcloud/index.php - line 37:
OC::handleRequest()