Error details are written to storage/logs/. Admin → System check shows the latest entries and the state of your server.
Check that the PHP version is 8.2 or newer. On Apache, make sure .htaccess was uploaded; it is a hidden file.
URL rewriting is off. On Apache, enable mod_rewrite and AllowOverride All. On NGINX, use nginx.conf.example.
Platforms change often, and some block data-centre IP addresses. Try these in order:
Make sure you are on the latest version (see Updating to a New Version).
For X, Facebook and Instagram, add cookies (see Adding Platform Cookies).
Add a proxy in Admin → Proxies and turn on Use proxy for that platform in Admin → Platforms.
If it still fails, open a ticket with the video URL.
Your host limits how long a script may run. Turn on Bandwidth saving mode (Settings → Monetization) to send visitors directly to the file.
Set 'canonical_host' => 'new-domain.com' in config/config.local.php.
Delete storage/installed.lock and open install.php again (upload it again if it was deleted).
Most pages are cached. Use Clear cache in Admin → System check.