did you guys find anything regarding this issue? I've the same error when I'm trying to add a media-file or even when I'm tying to open the media manager.
In the apache error.log I found the same error-message:
[Thu Oct 04 14:01:33.791508 2018] [:error] [pid 2382] [client 10.0.0.65:55349] PHP Fatal error: Call to undefined method PostInput::filter() in /usr/share/dokuwiki/lib/exe/ajax.php on line 18, referer: https://<my.local.domain>/dokuwiki/doku.php?id=start&do=media&ns=
line 18 from the ajax.php says:
$call = $INPUT->post->filter('utf8_stripspecials')->str('call');
Another thing is that behind the "ns="-parameter nothing is showing up in the error.log which maybe could also be the issue.
Login or editing articles works just fine.