I don't encounter that issue myself on 4.0.0-b5, but the error you receive is very generic, shown when no known error is available. This means there is a wide range of possible reasons for the error, from server or web server configuration, php configuration, htaccess configuration, to custom code that may interfere with the it.
It would be useful to know exactly what information you're using to create the recurring invoice, what URL you are sent to on error, what version of Blesta you're using, if you have any custom code/plugins in your installation, and what versions of php, MySQL, web server, and OS you're using.