After "Relaunch to update" BookyAI says it could not restart itself. What do I do?

From version 1.9.19, if BookyAI is still running fifteen seconds after "Relaunch to update", the panel says "BookyAI could not restart itself" and offers "Download the update": it opens the page with the newest version, which you install over the current one. Your books and settings stay where they are. On a Mac the usual cause is that BookyAI is not in the Applications folder; the panel then says "Move BookyAI to Applications first" with a button that does it. When a check or a download fails, the panel says why in plain words (no connection, a damaged download, no permission to write where the app is installed, no free disk space) and "Check again" tries once more.