Further investigation, by way of enabling 'Log Encoding' in the Admin Panel reveals some of these clues in your '/cc-core/logs/converter.log'
'Unable to connect to database'
or
<!DOCTYPE html>
<html>
<head>
<title>Page Not Found</title>
...
These symptoms indicate you're experiencing what we call the 'Alternate Location of PHP' or 'ALP' bug. This bug occurs because the default PHP binary detected by CumulusClips on your server was not CLI, most likely was PHP CGI or something else instead. These other PHP binaries do not behave properly in a command line environment, like that in which CumulusClips converts videos. This in turn leads to the problem you're having.
The solution luckily is pretty simple. You must find the alternate location of the PHP binary for use on the command line. If you're using one of these environments here are typical locations:
CumulusClips supported shared hosts: /usr/local/bin/php
Xamp on Linux: /opt/lampp/bin/php
Mamp on Mac: /Applications/MAMP/bin/php/phpx.x.x/bin/php
If you're not in any of these environments, then simply execute this from the command line:
whereis php
This should return the path(s) to any PHP binaries on your server. Next verify if its CLI compatible by exectuing the binary with the '-v' switch, i.e.:
/usr/local/bin/php -v
If the result of that command is something like this:
PHP 5.2.4 (cgi) (built: Apr 17 2009 14:31:37)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2009 Zend Technologies
Notice the '(cgi)', then that binary is no good, however if the result contains '(cli)' instead then you've found the correct binary.
Once you found the alternate path to PHP, add it to the 'PHP Path' field located at: Admin Panel -> Settings -> Video and you should be all set.