In 99% of the cases, Cloudflare chokes on a base64 posted string coming from Javascript's toDataURL() resulting in a "502 Bad Gateway" error. In some rare occasions the post succeeds, but for most of the time the error appears.
How can I work around that?
Below the actual POST variables. Note that the base64 string has been shortened for demonstration reasons (otherwise it would be a very very long blob).
action=createCanvasImg&q=loremipsum.png&hash=cd09a136415d11542410f62eb89c6221&string=data%3Aimage%2Fpng%3Bbase64%2CiVBORw0KGgoAAA
Update 1: The host is Mediatemple
Update 2: I posted a more general "Bad Gateway" question on Stackoverflow, so this problem may not be related to the specific case described above.
"disabled it" isn't an answer. If you didn't already contact CloudFlare's support team with full details please do that so we can actually investigate the issue.