https://wordpress.org/plugins/health-check/
https://inpsyde.com/deutsche-hoster-und-php-version-wechseln-so-gehts/
https://sebastian.expert/fix-wordpress-an-error-occurred-in-the-upload-please-try-again-later/
https://schnurpsel.de/wp-content/uploads/2009/12/memory-test.zip
Problems with https connection to wordpress.org: Check https connection to wp.org (included in Health Check)
http://dd32.id.au/uploads/2013/10/https-debugger.zip
https://wordpress.org/plugins/background-update-tester/
https://wordpress.org/plugins/dashboard-directory-size/
Alternative for supportdetails.com
Contains more information, and is a bit cleaner.
https://www.whatsmybrowser.org/
Builtwith.com analyses the used technologies of a website
https://builtwith.com/