Update 09/19/2019: The alert from 10th of September was probably a false alert.
(The client reporting the issue had other Wireless MIDI issues after updating to iOS 12.4.1. His iPad configured as MIDI client sent unexpected additional MIDI events to the server. I could not reproduce the issue.)
Update 07/28/2019: The problem is resolved with iOS 12.4!
Since the update to iOS 12.2 some users report, that SongBook+ freezes sometimes or cannot be started anymore.
This is a bug of operating system iOS 12.2 and 12.3.
When a Wireless MIDI connection is unexpectedly closed, e.g. because the WiFi router is switched off or the WiFi connection is lost, the MIDI function of iPas is completely blocked. This also blocks SongBook+ and even prevents to restart SongBook+. In this condition it is also no longer possible to start many other apps that use MIDI.
This blocking condition can only be resolved by a complete restart of the iPad.