Unfortunateley the update to iOS 10.3 is not without any problems.
Here you can find a list of currently known issues. We will update this article ongoing with additional information and solutions.
Problems with filenames containing german umlauts or special signs
The change of the file system (APFS) that comes with iOS 10.3 causes problems with files containing german umauts or other special signs (or other non-ascii letters from other languages). These files currently cannot be opened and displayed.
Delete the concerned files with iTunes from SongBook+ and then copy the files using WiFi sharing into SongBook+.
Please update to the latest version of SongBook+. The Problem is resolved with release 3.1.4 or later.
Update 08.04.2017: The problem seems to exist only with filenales containing german umlauts or special signs (or other non-ascii letters), if they are added after the iOS 10.3 update with iTunes on a Windows computer. Files that existed already before the update don't have that problem and files that are added using iTunes on a Apple computer also don't have that problem. ktionieren ebenfalls.
Update 12.04.2017: An update for SongBook+ (release 3.1.4) that will fix the problem is already uploaded to Apple. It will be online in about a week, after it was reviewed and released by Apple.
If you find other issues of SongBook+ with iOS 10.3, please report them to us.