I have a multiserver farm and on one WFE the CEWPs display fine, but on the other servers they show up on all subsites as :
Web Part Error: One of the properties of the Web Part has an incorrect format. Microsoft SharePoint Foundation cannot deserialize the Web Part. Check the format of the properties and try again.
Also on all servers if I try to add a CEWP I get a popup with the same message as above.
Another possibly connected symptom is on the root site I just get an Error with a Correlation ID. The only thing that seems useful from items with the correlation ID is something about a command cannot be executed, which is
"c:\windows\microsoft.net\framework64\v2.0.50727\csc.exe" /noconfig /fullpaths @"C:\users\temp\appdata\local\temp\xxxxx.cmdline"
I can type in a URL to any subsite and the pages will render, but all CEWP are replaced by the error message.