Many People face problems in resolving the Internal Server Error after successful installation of Dspace in Centos or Debian or Ubuntu.
Test that your Solr is working: http://localhost:8081/solr/
Here is the short cut solution to this problem. I have tested the solution on Centos 7 OS.
Go to your Dspace directory for the apache-tomcat-7.0.35 and then find the server.xml file using the command:
# nano /dspace/apache-tomcat7.0.35/config/server.xml
Then search for the line for Dspace Directory where solr is quoted and modify it with your installed dspace directory
Save the file and restart the Tomcat service.
Enjoy the error free world
N.B.: Please comment if this post is helpful to serve your purposes.