If it ever happens that SongBook suddenly and unexpectedly loses the full version, please call the "Restore In-App Purchases" function. This will unlock the full version again. An internet connection is required for this.
The function is displayed directly below the message that there are too many songs for the full version in the database.
Alternatively, you can also find the function in the menu at the "..." button.
If you bought the full version of SongBook+ by subscription, this can happen when the already paid period expires and needs to be renewed. With the 5-year version this should not happen.
If you are using multiple books, you may need to turn the multiple books feature back on afterwards to see all books again. To do this, open the book cover where the books are usually seen, press "Edit books" and then "Use multiple books".
On February 8, this suddenly happened to very many customers, even with 5-year full version.
The problem has arisen because the purchase receipts issued by Apple until recently unfortunately expired on February 7, 2023. Now new purchase receipts issued by Apple are valid until the year 2030. The "Restore in-app purchases" function fetches a new purchase receipt from Apple this way.
(Technical background information: https://developer.apple.com/news/?id=ytb7qj0x, https://developer.apple.com/support/expiration)