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I'm trying to debug a strange issue that happens on one my plugin's users system. The issue: When i submit my Plugin's Settings form (option-general.php?page=pluginname), I get a 500 error message. Other setting pages work fine, just this one.

I couldn't reproduce the error when simply using my plugin on a fresh wordpress copy. I have a local copy of his wordpress database + theme and plugins and now the issue occurs. I desactivated all its plugins (36!) and switched back to the default Twenty Fifteen Theme. The issue remains.

In Apache error log file tells me this:

"fastcgi: incomplete headers (0 bytes) received from server"

So, I changed my PHP mode from "CGI" to "Module".

And now, it works flawlessly, the issue has disappeared.

I then switched back to CGI, and ... It works still.

What may have happened here? I'm worried of not understanding what is the cause, since this plugin of mine is opensource and will run in many different setups.

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500 errors can be for many reasons. In context of submitting form they are more common because of security setup, typically mod_security. It tends to have strict rules for form submissions / POST requests.

There isn't much you can do about it really. At one host in the past I needed to ask support to disable some mod_security rules, because I couldn't even save some of my WordPress posts.

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It turns out it was a bug after all: one of my plugin options is a textfield in which the user can specify a message. It is provided with a default value, but that user chose not to have any message, so emptied it. It looked ok on the backend (field looked empty), but when i looked into the database it had become zillions and zillions of "\" signs! This created a memory log issue.

I feel stupid for forgetting to sanitize the input. I've patched it.

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