I've been doing several JIRA Portfolio demonstrations lately, and yesterday I encountered the "The timeframe of the scheduled items in your backlog exceeds the limit of the planning horizon" error during a client presentation (Oops!) Luckily, I had other Plans available, so I was able to carry on, but it did look weird. After playing around with the broken plan this morning, I was not able to find the error. I reported it to Atlassian. I will update this post once I have a solution.