When I log off from Joomla and log on again with the development off in Cloudflare, the same problem occurs. But there is no error on the screen. When I put Cloudflare in Development mode, the problem doesn't occur anymore. When thereafter I put Cloudflare out off development mode, the problem doesn't occur again.
At the moment, Cloudflare is out of development mode and I don't have any problems. But I can remember that indeed there was an error screen concerning the connection. As soon as the error screen reappears, I will make a screen copy.