After the upgrade of this site to Apex 18.1, I had an issue with the css created by Theme Roller.
On my development VM everything worked fine, but on my (run-time only) production VM a 404 error occurred for file /ords/tedstruik/r/25384/files/theme/42/v70/5864143227142593.css
This may have been a problem before the 18.1 upgrade; I never used Theme Roller before that.

Note: Something similar happened during a new installation of Apex 19.2 for my new OVH server, but now I got a 503 error. The solution below fixed that one as well.

I found the fix here: https://stackoverflow.com/questions/46073241/oracle-apex-theme-css-fails-to-load-through-ords?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
"After some time I found solution with my colleague. Running this command in webapps directory and restarting Tomcat solved our problem. Hope this post will help someone in the future."

On my VM I ran these commands:
As oracle: cd ~/tomcat/webapps/ java -jar ords.war validate --database apex As root: service tomcat restart