This error occurs when there are spaces in your URL.
What To Do
Examine your button or link URL and look spaces when you have cut and pasted the button code from another source.
A Conga button, when launched, is essentially a URL being passed between servers. Spaces in the URL will cause the "string" to break. Use Notepad or a similar text editor when copying and pasting buttons in your Salesforce org to avoid formatting errors.