When I'm scheduling a build via the "schedule build" button, nothing happens. A reload also shows no change. A build is not scheduled. There is nothing in the log indicating an error. Scheduling a build manually via URL (wget .../jenkins/job/Project/build?delay=...) works. This is in a reverse proxy configuration; Crumb issuer is configured as "Default Crumb Issuer", proxy compatibility is enabled. The setting mentioned in JENKINS-25116 ("Prevent Cross Site Request Forgery exploits") doesn't seem to exist anymore in this version.

With the web developer console, I might have found something relevant. It seems the POST request is issued via http, and therefore seems to be blocked by Firefox as mixed content (we're using exclusively https). I checked against a different installation of Jenkins, same platform, same versions of Jenkins and plugin, same settings regarding csrf, though Oracle JDK 1.8.0_171-8 and older Debian (8.11). There the request is https, and everything works nicely. Any things I could check, any settings which could induce that behaviour? Thanks in advance.