This occurs independent of the Minecart Experiments being enabled.

Steps to reproduce:

1. Build a nether portal.
2. Place normal rails in front of it so minecarts can be sent through.
3. Summon a big amount of minecarts on one rail (with a command block, for example).
4. Push the minecarts so they go through the portal.
5. Wait a few seconds (may be not needed).
6. Go through the portal yourself.
7. Now the "Stop: Invalid name parameter." error often occurs in the console. I have also seen "Cleanup: Invalid name parameter."