PHP/Apache POST limit?

Why don't you double check the size of the POST request. For example, you can use netcat to listen on 8080.

netcat -l -p 8080

Then, redirect your browser to use a proxy on port 8080 just before submitting the form. You should get something like this:

POST http://example.com/path/to/my/wiki/index.php HTTP/1.1
Host: example.com
Proxy-Connection: keep-alive
Content-Type: application/x-www-form-urlencoded
Content-Length: 217

The most important line is Content-Length and just double check the actual length of the content. That will at least help you to determine where things are going wrong. You will at least have an idea of the actual content size.

Then, ensure that you are directly connecting to Apache and not through either a proxy or a reverse-proxy. Some reverse-proxies place a cap on the maximum size of a request as a sort of security measure. So, you may want to check that as well as your Apache logs to ensure that nothing else is going on.

I had this same error and it was from mod security related to ajax uploading 500MB and less files, to fix I opened the config.

sudo vi /etc/httpd/conf.d/mod_security.conf

Changed this

SecRequestBodyLimit 131072 
SecRequestBodyNoFilesLimit 
SecRequestBodyInMemoryLimit

To:

SecRequestBodyLimit 524288000
SecRequestBodyNoFilesLimit 524288000
SecRequestBodyInMemoryLimit 524288000