this ticket can be closed ;)
I had an error according to #18134 - I changed the database prefix and got the error besides a white nomore accessible site:
Fatal error: Cannot access property started with '\0' in .../libraries/joomla/registry/registry.php on line
According to tickets response I checked the configuration.php and got it. There were no whitespaces, but I saw the encoding changed.
Set it back to UTF8. Problem solved.
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