I have a server which is refusing to run any Sandbox web parts with the error
"An error occurred while attempting to add the item to the page"
Steps already performed :-
- These web parts run fine on other servers.
- Normal non-sandbox web parts work fine.
- The SharePoint 2010 User Code Host is running.
- Nothing interesting in ULS lots - just "WebPartAdder_ItemCannotBeAddedULS"
- A reboot has been performed and appropriate incantations murmured.
This sort of error with Sandbox Solutions appears fairly frequently on the newsgroups but with little sign of resolution.
What steps would you take to troubleshoot an error like this?