Running into this same issue, both of those problems seem to be caused by the launcher not being able to pull the update file because it doesn't exist at the URL its looking for.
"Attempting to download (URL was here)
java.util.concurrent.CompletionException: java.nio.file.FileSystemNotFoundException: C:\Users\Andrew\AppData\Roaming\FTBA\instances\1c74c431-6ed0-4c53-aac3-9c0009177e50\installer.jar"
I created an issue on the GitHub page and was told its fixed and it should just be caching that's preventing it now. It's been an hour and I'm hoping it updates sooner rather than later.