403 Forbidden error

[Hans]

Suddenly this morning on one of my clients websites an Apache 403 error occurs:

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Forbidden

You don't have permission to access / on this server.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
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How can i solve the problem and how is it possible that this error suddenly occurs?

[Hans]

I found the clue:
The clients site exceeds the traffic limit.
ISPConfig blocked all access automaticly!