I just downloaded SmartRace to run it in a Mac Mini with El Capitan as OS. Restoring from a backup (created on my MacBook) worked fine, but the add-ons I bought can not be loaded. In fact, there is no button the reload previously purchased add-ons. This is working fine on my MacBook (running Mojave), but not on my MacMini.
This seems to be a bug. I assume that the debug file won't help you here. If so, please let me know and I'm happy to provide it to you.
Thank you for your support,