Android: Unable to instantiate activity / ClassNotFoundException

I solved this problems by selecting :Project->properties->order and export

select the all external jar files.clean and build it solved the issue

Since ADT update to revision 22 (May 2013) you have to check "Android Private Libraries" check box in Project -> Properties -> Java Build Path -> Order and Export in Eclipse for your older projects to get rid of this exception ...

I just got the same error (Unable to instantiate activity...) with Opera Mini. Opera Mini was on SD card (moved to SD card in the app setting). The error seems to be related to the fact that I swapped the SD card yesterday. The device was shutdown, I copied all data from the old card over to the new card (with cp -a) and then inserted the new card and started the device again. Everything seems to work as expected, but I see now that all apps on the SD card crashes with the same error.

• Device: HTC Desire HD (Android 2.2)
• Old SDHC card: SanDisk 8GB class 4
• New SDHC card: Kingston 16GB class 4

So I would say that this is a an Android bug and not something that can be fixed by app developers.

Also see:http://android-developers.blogspot.com/2010/07/apps-on-sd-card-details.html

It has always been the case that when you swap SD cards on an Android device, if you physically copy the contents of the old card to the new one, the system will use the data on the new card as if nothing had changed. This is also true of apps which have been installed on the SD card."

This seems to be incorrect.