You have data stored in an older format and/or unreadable data.
It appears that your reverse proxy set up is broken.
Your configured root URL does not contain the contextPath ("/jenkins").
Building on the built-in node can be a security issue. You should set up distributed builds. See the documentation.
Jenkins can enforce Content Security Policy (CSP). CSP tells web browsers what they are allowed to do while rendering a web page. This limits or even eliminates the impact of vulnerabilities like cross-site scripting (XSS). CSP is disabled by default for backward compatibility, but it is recommended to enable it, if possible.
