We goofed! In the previous version 7.5.6, we’ve added a few additional checks of the license key validity and it turned out one of them was one too many: it caused a message asking the user to enter a valid license key to appear in some circumstances, even though the correct license key was already entered.
To correct this error, we’ve released an updated version 7.5.7 today. (It has no other improvements over 7.5.6, so if you don’t see that message, feel free to skip this update). If you do want to upgrade, you can install the new version without removing the previous one, and it should upgrade the files and retain your settings.