If you import very big size of document from Google Docs, or adding too many photos in a note, RainbowNote runs out of memory and quits.
When you launch RainbowNote afterword, it tries to open the last note, and runs out of memory again.
To Solve this problem, open "Settings" from your home screen.
In Settings, find "RainbowNote", touch to open it.
Set "Reset Last View State" button ON. This button forces RainbowNote to open in list view mode when you launch it next time.
In rare case, just after upgrading iPhone OS, all paid apps can't launch. To solve this problem, try to download a free app from iPhone/iPod touch's App Store. Then your iPhone/iPod touch get authorized, and you can launch your paid apps again.