Error 500-Service unavailable: how to solve

Many DSpace users got Error 500. To solve the error, check the following things one by one.

Solr

Apply the following commands to restart Solr.

su dspace
/opt/solr/bin/solr start

exit

Tomcat

Apply the following command restart Tomcat

sudo systemctl restart tomcat9.service

Start the user interface

Apply the following commands,

sudo su
pm2 start /home/dspace/dspace-angular-dspace-7.4/dspace-ui.json