Magento – Magento 2.1.2 installation readycheck failed

At my clients webserver I'm trying to install Magento 2.1.2.

We have secured the domain with a Let's Encrypt SSL certificate and this seems to work fine.

Unfortunately I get two errors in the readycheck of Magento 2:

PHP Version Check Your PHP version is. The required PHP version is.
Hide detail

Download and install PHP from www.php.net using this PHP
Documentation.

For additional assistance, contact your hosting provider.

and

Cannot determine required PHP extensions: Warning: is_dir():
open_basedir restriction in effect. File(/etc/pki/tls/certs) is not
within the allowed path(s):
(/home/devcbs/:/tmp/:/var/tmp/:/usr/local/php56/lib/:/usr/local/php54/lib/:/usr/local/php55/lib/:/usr/local/php56/lib/:/usr/local/php70/lib/:/usr/local/lib/php/)
in
vendor/composer/composer/src/Composer/Util/RemoteFilesystem.php
on line 914

PHP Extensions Check missing PHP extensions. Hide detail

The best way to resolve this is to install the correct missing
extensions. The exact fix depends on our server, your host, and other
system variables. Our PHP extension help can get you started.

For additional assistance, contact your hosting provider.

To get through the readycheck I edited composer.json and added "disable-tls":true. This fix will get me through the readycheck but does not solve the problem, because it causes more errors after the setup has been completed.

Edit: I've tried to readycheck Magento 2.1.2 setup on another webserver without any problems. On this server there're no cron jobs set so far.

Does anyone know what kind of problem with the webserver might be and better how to fix this?

I hope someone can help me out with this because I'm clueless.