Image File Upload from Mac (client and browser) fails - everything else works

Nextcloud version: 13.0.0
Operating system and version : MacOs (not sure exact version but High Sierra something)
Apache: 2.4.6
PHP: 7.2

Image files (jpg and png) fail to upload from the Mac - either via the folder sync client or via the browser (tried Chrome and Safari) Images from any other machine, android phone, via various browsers or clients work just fine. Non-image files from the mac also work fine via the client or the browser.

The NextCloud Log contains no errors relating to this (just some trusted domain warnings from hours before)

The Apache Log shows the PUT Requests successfully and no errors:

[06/Mar/2018:20:58:03 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-0 HTTP/1.1" -
[06/Mar/2018:20:58:04 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-1 HTTP/1.1" -
[06/Mar/2018:20:58:05 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-2 HTTP/1.1" -
[06/Mar/2018:20:58:06 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-3 HTTP/1.1" -
[06/Mar/2018:20:58:07 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-4 HTTP/1.1" -
[06/Mar/2018:20:58:07 -0800] 10.20.30.15 TLSv1.2 ECDHE-RSA-AES256-GCM-SHA384 "PUT /remote.php/webdav/Friends/20170705_134011.jpg-chunking-2055461088-6-5 HTTP/1.1" -

Any suggestions?