After upgrading Jenkins to 2.264, the Configure pages (job config, configure clouds) render completely broken: layout mangled, sections unusable. This was the tables-to-divs UI modernization in Jenkins core 2.264, which broke configuration pages of plugins that still used the old table-based markup. The EC2 plugin and branch-api were among the culprits. The core upgrade itself is fine; the breakage comes from outdated plugins rendering config forms.
Jenkins 2.264: Configure UI completely broken after upgrade
Update the offending plugins: for the EC2 plugin you need at least version 1.54 for it to work on Jenkins >= 2.264, and similarly update branch-api and other plugins that render configuration pages. The breakage is plugin-side (old table markup vs the new div-based layout), so upgrading Jenkins core alone does not fix it; the plugin updates do. This was tracked as JENKINS-64072 and closed with resolution fixed.
Source: https://github.com/jenkinsci/jenkins/issues/11792
Source: https://github.com/jenkinsci/jenkins/issues/11792