Same problem here.
I have an iPhone 4s which worked great with the HDMI adapter.
I have read a lot of articles saying how iOS 8 is crashing iPhone 4s, but I didn't have any problems and was enjoying the new OS just fine.
Then I tried to play a movie and had the "incompatable with this device" message.
I immediately reverted back to iOS 7.
I followed the instructions listed below in the link. You may be able to do it too if it's not too late.
Unfortunately, I backed up my iPhone 4s while I had iOS 8 installed, and wasn't able to restore my iPhone 4s with the backup.
iCloud info like contacts and calendar was still intact though, thank goodness.
It was a lot of work to get everything working again, as I use two step authentication on my accounts.
I am kind of glad it happened though, it gave me an opportunity to clean house on the iPhone.
I hope apple fixes this problem pronto, I did like some of the new features on iOS 8, but I need to be able to show my movies off my iPhone.
Here is the link to revert to iOS 7. I hope its not too late for you if the HDMI adapter compatibility is as important to you as it is to me.
http://www.gottabemobile.com/2014/09/17/downgrade-ios-8-to-ios-7/