I did the 2.8 update, but the only response from the server is a 500 error, with the page “This page is not correct” which has the error gif and " We’re sorry, but something went wrong on our end."
How do I see what the exact error is on my server?
journalctl -xf -u mastodon-*
Is not showing me any errors, just times when the services started up.
All services are running. So I did
ruby -v
which gave me
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
So, my ruby version was outdated.
I then ran
$ RUBY_CONFIGURE_OPTS=--with-jemalloc rbenv install 2.6.1
$ rbenv global 2.6.1
$ exit
$ systemctl restart mastodon-*
But the error page still persists.
Weird 
Can i know exactly steps you have taken to update your instance?
and start all services one by one
systemctl start mastodon-web && systemctl start mastodon-streaming && systemctl start mastodon-sidekiq
So went through the install steps again one by one, and got it to work using these commands exactly: (the major part was restarting the services one by one instead of using restart with a wildcard)
su - mastodon
cd ~/live
git fetch --tags
git checkout $(git tag -l | grep -v 'rc[0-9]*$' | sort -V | tail -n 1)
bundle install --deployment --without development test
RAILS_ENV=production bundle exec rails db:migrate
exit
systemctl stop mastodon-*
su - mastodon
cd ~/live
RAILS_ENV=production bundle exec rails assets:precompile
exit
systemctl start mastodon-web && systemctl start mastodon-streaming && systemctl start mastodon-sidekiq