A NeoGAF thread posted by Nampad about the issue has uncovered several simple solutions to the problem. The easiest of these fixes (and the one IGN used to fix its own PS Vita) is detailed below:
- On a web browser (PC/Mac, tablet or phone -- even the Vita's own browser should be able to access it,) go to the Sony Entertainment Network website at https://www.sonyentertainmentnetwork.com/
- Click the "Sign In" button.
- Type in your PlayStation Network login email address and password.
- On the next page, you are asked to review and accept the Sony Online Network Terms of Service. Click to agree to the new terms.
- Go back to your PlayStation Vita's PSN login screen (or use any PSN feature such as Messages or Friends) and login to the network.
- Go to the Settings bubble on your PS Vita (the green suitcase icon.)
- Click Start to load settings.
- Tap the PSN option.
- Click "System Activation" and run through the "Game" and "Activate" menu choices.
- Close Settings and open the Friends or Messages app.
- Accept the ToS pop-up that comes up while the app is launching.
Sony could not be reached for comment at this time about the bug, but we expect a SEN login change or a quick System Update in the coming days.