How to clear Eclipse p2 repository cache

I am facing the puzzling fact that the information of update sites fail to be updated despite my forcing a reload in Preferences > Install/Updates > Available Software Sites.

I have a local update site (file:/ protocol, on Windows) and an online update site (https://) that I use as staging/test update sites for an open source project that I am maintaining.

I build the update site using an update site project that is stored locally and wiped clean each time I build it. When I have tested the new release in a different Eclipse instance and I have validated my changes, I then upload the entire update site to my server. Then, just to simulate what a user would do, I update the plugin in another Eclipse instance that runs on a different physical machine.

I have (yesterday) built another version, 2.2.0.201702052007 and uploaded it to my server. The previous version was 2.2.0.201702042059.

The problem that I have is that the Eclipse instances (Mars.2 and Neon) on my development machine keep reporting the previous to last version, despite my reloading the update site information. However, the other machine sees the new version without a problem.

This is what I've tried:

Reloading the information of the update site: each time, I get a confirmation message saying "information for [...] has been reloaded from the server" but it turns out that it hasn't been reloaded: I see the older feature version.
Accessing the update site from a different Eclipse instance on a different machine: I see the new version.
Loading the update site's site.xml file from a browser: I see the new version.
Using FileZilla to download the entire update site to a local folder and unzipping content.jar and artifacts.jar so that I can read the XML files embedded in those JAR files: I see no trace of the older version.
Removing the update site, restarting Eclipse and adding the update site again: the problem was still there.
As a last resort, I removed all files of the update site from the server: Eclipse still reported successfully reloading the information from the server.
I shut down the httpd service on the VPS. Eclipse reported success until I restarted it and it then failed. But once the web server was again online, it failed to actually send a request to the web server as it kept saying there was no update site! As a consequence, the online update site now appears empty and restarting Eclipse does not change that.
[EDIT] Even more incomprehensible, the Reload button reports success even when there's no network connection to the update site (network interface disabled).[/EDIT]

There seems to be in the provisioning framework a cache somewhere between the UI and my server that reports an outdated information and feature version in spite of the explicit requests to reload that very information.

Is there any file or folder that I can delete to have the provisioning framework reset itself? If possible, I would altogether disable its cache.

Solution

I've found out that Oomph apparently has an action on the update site information retrieval process.

Anyway, I could recover normal operation (for now) and have the information properly reloaded by first deleting the appropriate files in C:\Users\...\.eclipse\org.eclipse.oomph.p2\cache.

By “the appropriate files”, I am referring to the fact that files in that folder are named after the URLs of repositories known to your Eclipse instances.