I started having this or a similar problem a couple of weeks ago.
For several months, I used my Amplifi 150 and iPhone app with no connection problems. Then one day, I opened the app and it immediately flipped over to a big "red" sceen requiring me to first log into my Line 6 account. I tried logging in but it wouldn't recognize my account info. I turned everything off, rebooted both the amp and iPhone and then it would let me log in and connect the app to amp. The next time I used the amp, the same thing happened. Eventually, -- no matter how much rebooting I did -- it wouldn't let me in at all. So, feeling at a wit's end, I deleted the app from my phone and reloaded it from the app store. Surprisingly, it worked fine after that! I was afraid I'd lose all my settings and tones, but at least in my case, they were all still there. I haven't had a problem since then (fingers crossed...) AE