Hey there! It's likely the .htaccess error you're seeing is likely related to the problem, but that isn't the root cause of the 500 error on the system. There are likely other things being logged on the server by the time you check the error logs in WHM, so the 500 err is not being displayed.
I always like to run this command on the server while refreshing the page, which lets me see the 500 error in real-time on the server:
tail -f /etc/apache2/logs/error_log
You could try that from the WHM >> Terminal application as well, or you can scroll through the error log window you included in your screenshot to see if you can find more details on the error. I'd expect to see either "500" or "Internal Server Error" in the logs when the problem is happening.