Live Update failure with error 500

We have always used Live Update without any issues. Recently we get a PHP fatal error; but nothing has changed in the Contao configuration. According to my hosting provider nothing has changed in their setup either. The check-tool v12.0 reports that we can use the LiveUpdate service. But the file permissions cannot be tested, because the posix_getpwuid function has been disabled. So maybe there is a file-permission error?

The log-file shows:
Backend fatal error: PHP Fatal error: Phar::webPhar(): Failed opening required 'phar:///........./public_html/contao/update.phar.php/index.php' (include_path='.:/usr/local/php56/lib/php') in /......l/contao/update.phar.php on line 62\n,

Any suggestions welcome.

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Contao 3.5. / PHP 5.6.30

Update: The hosting provider enabled the "posix_getpwuid" function and I removed the update.phar.php file.

Check (v12.0) is now all green, you can use the Live Update.
A new update.phar.php is being downloaded automatically.
file permissons 0644; directories 0755.

Still the same PHP Fatal error: Phar::webPhar(): Failed opening required 'phar:///........./public_html/contao/update.phar.php/index.php'