Hi , can you please provide the full error log stack trace . It seems the the provided error log is incomplete . It feels like the error perhaps generated from elsewhere and bubble up to the app bootstrapping . Having the full error log will be very helpful if it is incomplete .
Also can you check your config.inc.php file to see if there is multiple entry of app_key ? or the current app_key being comment out like ; app_key = "base64:......" ? and it should be under the [general] section of config file .
As you have mentioned that you can see that there is an an entry like app_key with value and if that is not commented out , please run the command php lib/pkp/tools/appKey.php validate from you install path and share the details if there is any error .
Finally run the command php lib/pkp/tools/appKey.php generate --force to regenerate a new app key and force overwrite if there is any existing one to see if that resolve the issue .
Regards
Touhidur Rahman
PKP Dev Team