I have Sitecore 9.1 instance and today my sitecore license file expired. I had to replace the license.xml in multiple places to make sure my site, xConnect and Identity Server sites work.
- Sitecore site:
- The /App_Data folder
- xConnect site:
- /App_Data/jobs/continuous/ProcessingEngine/App_Data (new to Sitecore 9.1)
Hope this helps if you have license expiration message.