— It's obvious now that nobody on the forum knows how to solve this. If you think that Construct creates a key in the wrong format, which is no longer supported by Google Play, you need to log a bug report:
github.com/Scirra/Construct-3-bugs/issues
Otherwise you can try researching this issue in the internet how to generate a proper v2 key. And if you find the solution, post it here for the benefit of others.
My workaround was to change the target version to 29. Export as Cordova project, extract files and change target version from 30 to 29 in config.json and config.xml
Then package back to zip and build with C3 through Export Manager.
This is only a temporary workaround, I'm afraid Google Play will soon no longer accept SDK 29.