I’ll add another solution to the pile. /panel was redirecting to Kirby’s error page, and the problem was with file permissions.
I made sure all the files and folders had proper permissions and the problem went away. I find that this solution also fixes 500 errors in some shared hosting environments.
# Loop through all files recursively and change permissions to 644
find starterkit/ -type f -exec chmod 644 {} \;
# Loop through all folders recursively and change permissions to 755
find starterkit/ -type d -exec chmod 755 {} \;