Yesterday I've tested the plugin and it seemed to work properly. Now it doesn't enable anymore. Maybe needs more testing, I will keep you up to date.
https://pastebin.com/SwAKjAas

Oh, works fine for me, I just tested it.
Do you have any additional information on what you might have changed since it last worked?

Just created some flights, travels and stationary dragons and restarted the server. I will test it again soon. :)

Indeed the stationary dragon is the problem.

Latest Paper 1.12.2, latest DragonTravel.

1. Create a stationary dragon
2. Restart your server

The plugin won't start with the error in the first post.

Thanks for the report, I'll try to fix is asap.