When I log into my main Trello, I get a popup saying:
"You are approaching the limit on the number of lists you can have on a board. Please remove any lists which are unnecessary."
According to the documentation: https://trello.com/guide/trello-101
"There’s no limit to the number of lists you can add to a board, and they can be arranged and titled however you’d like."
So is it safe to assume the warning message is a bug, given that they explicitly document that there is no such limit? It'd really suck if it stopped working on me all of a sudden.
One note of interest: My board currently only has 17 active lists, but I do generate a todo list on a daily basis using the API, so I'm sure there are many hundreds of ARCHIVED lists on it.
I don't see an easy way to clean out my archived lists, aside from building a cleanup script that handles it through the API, so hopefully it's just a Trello bug that will go away on its own. My only other option would be to abandon ship and migrate to a new "main" board to work off of.