You receive lots of warnings in your logs with the following pattern:
WARNING: Found invalid crumb xxxxxxxxxxxxx. Will check remaining parameters for a valid one... hudson.security.csrf.CrumbFilter doFilter WARNING: No valid crumb was included in request for /ajaxExecutors. Returning 403.
The issue is related to JENKINS-40344. This bug materializes in the logs if you have CSRF protection enabled (which is a recommended practice). To learn more about CSRF Protection, you can click here.
In the meantime, and as a workaround, you can avoid flooding your logs with these messages by changing the log level to
SEVERE on the following logger
In order to do this, please follow the procedure below:
hudson.security.csrf.CrumbFilterin the textbox, select the
SEVERElevel value and then click submit.