Major cache, cookie or session issue after upgrade to 5.1.5

[DisasterDotCom]

Update...

The headers get sent down by vBulletin or Apache, but the only thing that's changed is vBulletin, and this started immediately after the change. It occurs with both Chrome and Firefox.

Looking at the logs:

1. I click on new thread GET /forums/new-content/44 HTTP/1.1
2. I make the post POST /forums/create-content/text/ HTTP/1.1
3. The page refreshes GET /forums/forum/general/test-forum/28305-this-is-another-new-topic
4. I add a reply POST /forums/create-content/text/ HTTP/1.1
5. The reply shows up POST /forums/create-content/loadnode HTTP/1.1
6. I go to the parent forum of the post GET /forums/forum/general/test-forum HTTP/1.1
7. I click on the post, page displays and a POST /forums/ajax/api/node/incrementNodeview HTTP/1.1 and then GET /forums/foru...-another-testt HTTP/1.1

Everything has a status code of 200. Item 6 and 7 have a Cache-Control: max-age=3600 and the date and time of the original post.

Isn't the process from the client to the server supposed to be something like... pull down the original page and cache it locally. Request the same page later, send a request to the server for a header to see if the page has a new time, if they match then pull the local copy, if they don't match send the new copy. Apache has no idea whether the page has been updated or not - that is generated by vBulletin, correct?

--------------- Added [DATE]1423090573[/DATE] at [TIME]1423090573[/TIME] ---------------

Alright... after 8 hours of troubleshooting, and Zachery (on vbulletin.com) mentioning that there are only two types of caching - on or off - I found this is not a vBulletin issue.

For future reference... IF you have vBulletin installed in a subdirectory (i.e. /forums), and the PARENT directory has an .htaccess file, the directives from this parent .htaccess file will be used. Any directives you have in the subdirectory will override those of the parent, but if there's something in the parent that's NOT in the subdirectory .htaccess, it will be pulled from the parent.

In our case, the parent .htaccess had the following directives (in addition to many others):

ExpiresByType text/html A3600
ExpiresByType text/richtext A3600

These were setting a 3600 second (60 minute) expiration on HTML content.

I've modified the vBulletin .htaccess file to be the following:

ExpiresByType text/html A0
ExpiresByType text/richtext A0

This effectively sets max-age to 0 so HTML does not cache.

We're still trying to figure out why this is corresponding to the upgrade to 5.1.5. Something had to change somewhere, because (as far as we know) nothing else changed.