Image upload not longer possible / Getting 504 error

I’ve a problem. After changing the DNS Server for my domain from Strato to CloudFlare, I’m no longer possible to upload any images via my WordPress page. I’m getting this error in the console:

Failed to load resource: the server responded with a status of 504 ()

In my server logs I can find this:

19110#0: *2032 upstream timed out (110: Connection timed out) while reading response header from upstream

I’ve changed nothing in my server. I’ve just added the CloudFlare root cert to my domains. I can’t find a solution…