During the development of some sandboxed workflow activities, the error like below popped up twice.
Error activating type from sandboxed code assembly. Activation did not throw an exception but it returned null. - this.UserCodeAssemblyGroupId = ", GroupId = "650D303F58024C8289180999F76F3C93-kFBMtpS5DYg4I0f3nKeZlPmPpSDkR4V7bpY2A5+UqYk="", assemblyName = "EFSBWFActivities.getCountByCamlSB, Version=184.108.40.206, Culture=neutral, PublicKeyToken=fa283f9d97b09f0f", typeName = "EFSBWFActivities.getCountByCamlSB"
The closest explanation I could find was here, but in my case, there is not relevant customized assembly stored in GAC. Since not many people complaining about this issue, I must made a silly mistake. Unfortunately, it cost me 7 hours to figure out the real reason: one error was caused by a typo in "ClassName" in elements.xml (I didn't realize that it was case sensitive), another one was caused by a typo in "namespace" in the .cs source code file.
Hopefully this can save you some time doing the trouble shooting.