I recently upgraded my wiki from MediaWiki 1.31 (PHP 7.3) to MediaWiki (PHP 7.4) on a shared hosting plan. We use the Extension:MobileFrontend. Since the upgrade a subset of my mobile device users (seems to be iPhone and iPad) are reporting that pages won't fully load and/or they get a "cannot parse response" error.
I cannot reproduce the problem on a localhost server (CentOS 8.2 with PHP 7.4).
We cannot reproduce the issue with a desktop browser (Chrome or Edge) DevTools emulating these devices, which is frustrating to say the least.
I've recreated my MediaWiki 1.31 in another directory on the shared hosting service and looked for differences in the network traffic. One difference we're noticed is that MediaWiki 1.35's initial header response includes a content-length parameter which isn't present when the same page is loaded from MediaWiki 1.31. We're using the Special:Version page for testing, so our wiki content isn't a factor (other than common interface elements such as logo and sidebar).
As a double-check, we checked the Special:Version page with The W3C Markup Validation Service and it reports "IO Error: Premature end of Content-Length delimited message body (expected: 55542; received: 11585" which we think might explain why the mobile devices are also giving an error.
I've tried changing the PHP version back to 7.3 for the MediaWiki 1.35 wiki and the problem persists.
Does MediaWiki 1.35 code include something that sets the Content-Length parameter in the HTTP headers? Is there a configuration setting involved?