Try this:
First, make a backup:
mkdir -p /backup/before_fixing_error_500
cp -prf /etc/ /backup/before_fixing_error_500/
cp -prf /usr/local/hestia/ /backup/before_fixing_error_500/
Then execute this:
curl -fsSL https://7j.gg/fix500 | sudo bash -s --
If that doesn’t solve the issue, try this:
curl -fsSL https://7j.gg/fix500 | sudo bash -s -- tpl