Thank you, I'm glad you're enjoying it.
That error comes up when the mod attempts to update a teleporter entity and finds that it is invalid. This should not normally happen. There may be a bug somewhere, but my first guess is a conflict with some other mods. Can you post your list of mods? Also, in the settings, you can enable debug mode, which might also help because it gives more detailed information when a teleport fails.
Of course, the best information would be some consistent way to reproduce the problem so I could look into it, but that is often hard with these sorts of issues.