Bad tokens is the servers way to tell you that something went wrong when loading a map/location most of the times. The client failing to communicate with the server, or other way around. If there was some connection problems just during the time you tried to load a new map, this might happen to you. Also maps that has an extreme amount of online users, or more objects and other custom things might take longer to load, and the chance of getting bad tokens might rise. Usually the issue doesn't last for too long and get solved by just trying going through a portal again or loading the character again or just keep reconnecting, for some it's worse than others.
I hope this answers your question!