Invalid Base URL. Value must be a URL or one of placeholders: {{base_url}}

If you’re seeing the following error in Magento 2

Invalid Base URL. Value must be a URL or one of placeholders: {{base_url}}

The problem may the lack of a trailing slash on your base URLs

http://example.com
vs.
http://example.com/

There’s some questionable URL validation going on in

#File: app/code/Magento/Config/Model/Config/Backend/Baseurl.php
private function _isFullyQualifiedUrl($value)
{
    $url = parse_url($value);
    return isset($url['scheme']) && isset($url['host']) && preg_match('//$/', $value);
}

Although, to be fair “private” usually means

I don’t know how to do this so I’m going to hack something together that no one else will use and/or call.

So whatever poor overwowrked dev got this assignment has that going for him.