A couple of months ago I started to experience random 403 "no valid crumb" errors through-out when using Jenkins. The errors show up particularly frequently when trying to replay jobs but they seem to happen on just about any page depending on the embedded content. I've updated Jenkins at least once since this started happening and numerous plug-ins have been updated as well. This current Jenkins instance has been working for a year or more without issue until just recently and I had a similar instance I used for numerous years before that. 

The problems aren't limited to a particular browser either (happens for sure in Chrome/Firefox)

The most common instance is after submitting a form to start a job (this is a rebuild but it happens when building a new job fresh also):

Sometimes the errors show up below various input fields:

Simply hitting the browser back button and retrying often works, but sometimes it doesn't. I've had it fail upwards of a dozen times in a row (usually I get frustrated and move on to something else for bit). This does not seem to happen anywhere but though the web UI, scripted access works without issue. 

When viewing console logs steaming in for various running jobs the browser will also sometimes just get stuck in some kind of rapid refresh loop trying to load the page content, the header and side bar display/disappear rapidly and the title in the tab disappears/re-appears rapidly.

I've also noticed that I (again sometimes, if I simply refresh the page it goes away) get a notice that reverse proxy is broken - Note: I do not have a reverse proxy configured, it's a pretty vanilla install of Jenkins on a Windows Server instance. 

I have worked with our IT department trying to figure this out, they can't see anything on their end that would be impacting things. While monitoring traffic to/from my laptop and the Jenkins instance, as well as the Jenkins instance to the internet, they're not seeing any blocked/suspect traffic. I'm told they also looked at the firewall and can't see that would impact anything. After several attempts to resolve the matter they requested I open this ticket and provide as much information as I can. 

Coles Notes:

• Occasional problem started happening on a previously functioning, quite basic install of Jenkins.
• Occurs on numerous pages within Jenkins when trying to run/re-run jobs
• Does not happen for scripted access, only within the UI.
• Not limited to one particular user
• Not limited to one particular browser
• Occasional message about reverse proxy being broken, no reverse proxy configured. 
• No obvious traffic being limited on a firewall/network level