Permissions for Mautic

permissions.sh

cd /var/www/mautic
find . -type d -exec chmod 755 {} \;
find . -type f -exec chmod 644 {} \;
chmod -R g+w app/cache/
chmod -R g+w app/logs/
chmod -R g+w app/config/
chmod -R g+w media/files/
chmod -R g+w media/images/
chmod -R g+w translations/
chown -R $USER:www-data .

Thanks for sharing this, but I did need to make few changes before it work for me.
I'm using Mautic version 2.15.1 (latest release).

cd /var/www/mautic
mkdir app/spool/
find . -type d -exec chmod 755 {} \;
find . -type f -exec chmod 644 {} \;
chmod -R g+w app/cache/
chmod -R g+w app/logs/
chmod -R g+w app/config/
chmod -R g+w media/files/
chmod -R g+w media/images/
chmod -R g+w translations/
chmod -R g+w app/spool/
chmod -R g+w themes
chown -R $USER:www-data .

Mautic 3

find . -type f -not -perm 644 -exec chmod 644 {} +
find . -type d -not -perm 644 -exec chmod 755 {} +
chmod -R g+w var/cache/ var/logs/ app/config/
chmod -R g+w media/files/ media/images/ translations/
rm -rf var/cache/*

Well that's...new.
Apparently, the php getting set to 8.0 was indeed the start of the problem. Then when I manually deleted the cache, since nothing could run correctly, something in the cache wasn't created correctly--no big surprise there. And you'd THINK that setting the php back down to 7.4 and deleting the cache would fix it--but it didn't.

However, when I deleted the cache the first time, I let it go to the trashcan. So having set the php down to 7.4 and manually wiped the cache again, I RESTORED this older cache, and now the site's working again. That's totally bizarre, but if makes me fear it's only working because there's something needed in the restored cache, and as soon as it's gone or I wipe the cahce I'll be dead again, so I'm going to zip the entire cache folder up for future reference/backup and try applying the latest upgrade and see what disasters I can invoke.