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.