Re: When I deployed my custom WebPart to page on WebSite I got the message in the webpart body:“Web Part Error: Sandboxed code execution request failed."

Hi Slavi,

In regard to the issue about the specific error message “Web Part Error: Sandboxed code execution request failed.”, this could be caused by various reasons. To solve it, please try the steps below:

1. Check if there is any server side code(i.e. need to be configured on the server side) included in the solution, if so, please remove the code and try again.
2. If you’re querying a large list, try not to loop all the items in the list, since this could resulting in performance problems and will be blocked by SharePoint Online.
3. Have you tried run the solution on an on-premises SharePoint server? How did that going?

Also I’ve found some posts that may be helpful. For details, please refer to the links below:

  A Lap Around SharePoint 2010 Sandboxed Solutions Resource Quotas

  Developing, Deploying, and Monitoring Sandboxed Solutions in SharePoint 2010

  Office 365 for developers