Read the official Contao announcements.
Contao 3.5.10 is available
by Leo Feyer – Announcements
Contao version 3.5.10 is available. The bugfix release fixes several issues, including issues with the search index, the book navigation and the back end user switching. In addition, the handling of IDNA e-mail addresses has been consolidated.
After the update, you have to rebuild the search index. This applies to both the manual update and the Live Update. If you update manually, you should also purge the search index before you apply the database changes.