Same problem even in Jenkins 2.263.4, apache-tomcat-9.0.43 and openjdk version "1.8.0_282 as well.

Solution tried :-

1) Under Configure Global security --> CSRF Protection -->Enable proxy compatibility( Tick marked Enabled). - Didn't work so disabled with below command.
2) hudson.security.csrf.GlobalCrumbIssuerConfiguration.DISABLE_CSRF_PROTECTION = true - Even after this also didn't work.
3) Installed the Strict Crumb Issuer plugin. Enabled this plugin and uncheck Check the session ID from its configuration (Under Jenkins Configure Global Security). 
4) Restated the Jenkins.

01-Mar-2021 08:12:10.604 WARNING [Handling POST /jenkins/configSubmit from 45.46.58.59 : http-nio-8080-exec-2] hudson.security.csrf.CrumbFilter.doFilter No valid crumb was included in request for /jenkins/configSubmit by sumit.mital. Returning 403.

Still same problem persists.

Jenkins > Manage Jenkins > Configure Global Security > CSRF Protection - Default Crumb Issuer tried by ticked and Un-ticked the Enable Proxy Compatibility.  But no success. still getting same error.

Jenkins > Manage Jenkins > Configure Global Security - Apply works. But Save results same error.

Even tried by addeding below in /apache-tomcat-9.0.43/conf/tomcat-users.xml file, however still same issue.

<?xml version='1.0' encoding='utf-8'?>
<tomcat-users>
    <role rolename="manager-gui"/>
    <role rolename="manager-script"/>
    <role rolename="manager-jmx"/>
    <role rolename="manager-status"/>
    <role rolename="admin-gui"/>
    <role rolename="admin-script"/>
    <user username="user" password="password" roles="manager-gui,manager-script,manager-jmx,manager-status,admin-gui,admin-script"/>
</tomcat-users>

My best suggestion is to not try to run Jenkins under tomcat. Run it as a separate application so that you don't need to wrestle with Tomcat configuration.

If you need a reverse proxy between the user and Jenkins, consider nginx, Apache, HAProxy, or Squid as described in reverse proxy configuration.

We have been running our jenkins under tomcat for last 10 years until we upgrade our jenkins from 2.235.5 (LTS) to 2.263.1-LTS we had no issue.

Sure we will consider to move out of tomcat.

Nginx reverse proxy side seems no issues, because i have tested without nginx reverse proxy even in that too facing same problem.

it looks CSRF is causing issue with tomcat. still digging. hopefully will sorted out.

Tested with Jenkins - 2.235.5-LTS and with same version of apache-tomcat-9.0.43 here there is no issue.

Hi Mohan,

Is this issue is resolved..? Please let me know what is the fix for this...?I'm facing same issue. It will great helpfull if you provide the solution..

Hi Mohan,

Is this issue resolved? if yes, please provide the solution. even am facing the same issue. It is taking lots of time to fix and am not understanding the root cause for this. This has been a blocker in the production environment,

Hi Suresh & Deepa, I have solved my problem by moving my Jenkins outside of Apache.