I contacted the hosting provider and according to them the ddos protection did not change the last weeks.
But they updated PHP on their servers.
Could that be the problem why I get now more often the "HTTP Status: 403 (Forbidden) - You don't have permission to access" message?
When I make updates and a few back-ups on several sites more or less the same time, I get this error.
And when I wait a minute or 5 after that error message I can normally create a back-up.