Some POST Requests Failing, GET is OK

What could be causing this issue?

[vordmeister]

I run a vBulletin forum and a couple of members have reported in the last week that they are having trouble with the features involving POST requests.

What are the things between visitors and the server that can break POST requests?

Nobody has trouble with GET - the forum is super-speedy for everyone who is just looking around, but a couple of members have reported that submitting new posts or using the other pages that require a POST request takes a few tries.

I've checked through the logs and all I see is the visitor making a single POST request which is dealt with in less than a second. I don't see any failures. I don't think the failed requests are getting as far as the server.

It's a dedicated server located in a decent datacenter with decent connections. Trace-routes from a number of members are all fine - nothing seems slow along the way.

We moved servers last year after most members reported similar issues. The new one has been perfect until last week.

I'm not sure where to look. It has to be something between the visitor and the server that is becoming unstuck when it has a little more data to process than a simple GET request.

Has anyone come across this sort of issue before? What should I be looking at?

[vordmeister]

I'm fairly sure I've posted in the wrong forum. Apologies to everyone (embarrassed).

I reckon it's a routing issue but I've failed to find any similar issues on the net. Mostly because I've not figured out how to disguise my search to avoid all the common mentions of the word POST. In any case it'll be the software in the routers rather than the hardware so this is the wrong place.

Might a friendly moderator move me over to Webmaster General?