The concrete error message was “premature end of file” and Apache’s error_log showed that there are syntax errors, even if there were none:
PHP Parse error: syntax error, unexpected T_STRING in /var/repos/...
As it seems, PHP files were interpreted instead of being treated as text as required for SVN due to default MIME types configuration. Adding this line the the virtual-host configuration file for the repository will prevent apache2 certainly from interpreting PHP code:
AddType text/plain .php
That’s four words – and one afternoon was gone.