Cause admin-ajax.php 403 forbidden

Hi @arshen;
Can’t be 100% certain, but this is probably rather a problem with page caching (wp super cache); typically admin-ajax.php requests use a “nonce”, a semi-random string which is generated and added to the page. When the ajax gets triggered it sends that “nonce” (together with the data it tries to post) to the server where the nonce gets checked. Now the problem; a WordPress nonce has a limited validity, typically (but not always) 4h. If the “nonce” does not check out, the request is refused, which is what you’re observing.

The solution; lower “Cache Timeout” on WP Super Cache’s “Advanced” tab. The default there is 3600s (= 1h), so either you have a (much) higher value there OR you have a plugin which has a _very_ short nonce lifetime.

Hope this clarifies/ helps,
frank