During the development of a Sandboxed workflow activity, I got error message below from ULS during the test.
Solution Deployment : Missing one or more of the following attributes from the root node in solution EFSBWFActivities.waitForUnlockWorkflowSB.wsp: assembly '', type ''.
At first I thought there was a typo in the elements.xml file or package file. But after comparing them with some other correct files, this possibility was excluded. Then I spent quite a while googling, and occasionally noticed there was some code such as "SPSecurity.RunWithElevatedPrivileges()" in the activity! "RunWithElevatedPrivileges" is not supported by Sandboxed code. After removing it, everything goes well.