What version of phpBB are you using?
Who do you host your board with? https://ovh.com
How did you install your board?
I used the download package from phpBB.com
What is the most recent action performed on your board?
Using HTTPS only. For that I have set the configuration in ACP and added a port redirection (80 -> 443) in my .htaccess file.
Is registration required to reproduce this issue?
I dont know.
Do you have any MODs installed?
What MODs do you have installed?
Paypal donation mod, board announcements, phpbbgallery (and its extensions).
What styles do you currently have installed?
Prosilver (by default) and a custom style I made (based on Prosilver).
What language(s) is your board currently using?
Which database type/version are you using?
Which PHP type/version are you using?
PHP 5.6 with FPM
What is your level of experience?
When did your problem begin?
When I changed the configuration (to use HTTPS only).
Please describe your problem.
Tons of requests are failing. Error messages are like this in my error.log file : [error] [client XXX.XXX.XXX.XXX] [host mywebsite.com] FastCGI: comm with server "/path/to/app.php" aborted: error parsing headers: duplicate header 'Content-Type'
And sometimes it says that it fallbacks to CGI (it is written just bellow the first error message, like this : [error] [client XXX.XXX.XXX.XXX] [host mywebsite.com] FastCGI: An error happend on Fastcgi processing, fallback to CGI, referer: https://mywebsite.com/could_be_any_page ... board_here
), sometimes not.
Do you know how can I fix this please?
[EDIT: You'll find my PHP and CGI/FCGI configurations bellow]