FWIW to anyone else (and I can't believe Roku hasn't resolved this issue in several months!): Through trial, error, and luck I found a fix. I'm using a Roku HDMI Stick and an iPhone 6s. Hotspot's name is very short and neither it nor its password use special characters or spaces. Tried rebooting both more times than I want to admit. Roku could see the hotspot but threw error 14.40 when trying to connect. The trick was to keep the iPhone displaying the hotspot config screen (the one that shows your hotspot password) through the ENTIRE connection process. I only thought of this test because my daughter's unbelievably antique iPad requires the same thing. Everything works, and I love my Roku, but shame on them for not paying more attention to this basic issue.
Thanks for letting us know about the issue you're running into. We're still investigating some reports of issues connecting to iOS mobile hotspots.
Please feel free to send me a private message with the Roku device's serial number from Settings>System>About, the Roku account email that the device is linked to, along with details about the iPad that you're encountering the issue on. Please include the model of iPad and the version of iOS that it is currently running. I'll follow up with you directly to continue assisting you.