I recently had to build a pretty sizeable form with approximately 65 fields. I created the form and performed the initial save with no issues however on returning to edit the form or creating a new form based on the template I was presented with the error message "Failed to save form". In true Umbraco style that's all the information I got, no reasoning and nothing in the logs and so began my quest to find a solution. Thankfully I was not the first to have the issue and I'm sure I won't be the last so I thought I make this post as a reminder to myself and to aid anyone else in their search for a solution to the same issue. The fix is nice and straight forward and simply requires the addition of the following entry in your web.config: [code language="xml"] <system.web.extensions> <scripting> <webServices> <jsonSerialization maxJsonLength="500000"> </jsonSerialization> </webServices> </scripting> </system.web.extensions> [/code] You can amend the maxJsonLength attribute value to suit your requirements but the above worked for me. Credit to @NielsHaasnoot for his solution to my problem.