Ich habe den Request aus Deinen Logs mal auf meine Testmaschine los gelassen. Dabei bekomme ich im Apache error.log folgende Fehler/Warnings:
[Fri Aug 24 09:29:18.157929 2018] [php7:notice] [pid 10198] [client ::1:55669] PHP Warning: imagecreatefromstring(): Passed data is not in 'JPEG' format in /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php on line 1080
[Fri Aug 24 09:29:18.157940 2018] [php7:notice] [pid 10198] [client ::1:55669] #1 /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php(1080): imagecreatefromstring('\\xFF\\xD8\\xFF\\xE0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00...')
[Fri Aug 24 09:29:18.157947 2018] [php7:notice] [pid 10198] [client ::1:55669] #2 /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php(922): EGroupware\\Api\\Contacts::resize_photo('\\xFF\\xD8\\xFF\\xE0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00...')
[Fri Aug 24 09:29:18.157956 2018] [php7:notice] [pid 10198] [client ::1:55669] #3 /opt/local/apache2/htdocs/egroupware/addressbook/inc/class.addressbook_groupdav.inc.php(675): EGroupware\\Api\\Contacts->save(Array)
[Fri Aug 24 09:29:18.157964 2018] [php7:notice] [pid 10198] [client ::1:55669] #4 /opt/local/apache2/htdocs/egroupware/api/src/CalDAV.php(1741): addressbook_groupdav->put(Array, '7b83a8eb-5efd-4...', 5, '/ralf')
[Fri Aug 24 09:29:18.157972 2018] [php7:notice] [pid 10198] [client ::1:55669] #5 /opt/local/apache2/htdocs/egroupware/api/src/WebDAV/Server.php(1840): EGroupware\\Api\\CalDAV->PUT(Array)
[Fri Aug 24 09:29:18.157980 2018] [php7:notice] [pid 10198] [client ::1:55669] #6 /opt/local/apache2/htdocs/egroupware/api/src/WebDAV/Server.php(306): HTTP_WebDAV_Server->http_PUT()
[Fri Aug 24 09:29:18.157987 2018] [php7:notice] [pid 10198] [client ::1:55669] #7 /opt/local/apache2/htdocs/egroupware/api/src/CalDAV.php(2053): HTTP_WebDAV_Server->ServeRequest(NULL)
[Fri Aug 24 09:29:18.157995 2018] [php7:notice] [pid 10198] [client ::1:55669] #8 /opt/local/apache2/htdocs/egroupware/groupdav.php(52): EGroupware\\Api\\CalDAV->ServeRequest()
[Fri Aug 24 09:29:18.158004 2018] [php7:notice] [pid 10198] [client ::1:55669] #9 {main}
[Fri Aug 24 09:29:18.158014 2018] [php7:notice] [pid 10198] [client ::1:55669] # Instance=boulder.egroupware.org, User=ralf, Request=PUT http://boulder.egroupware.org/egroupware/groupdav.php/ralf/addressbook/7b83a8eb-5efd-46c4-bb44-df8ceb20c80b.vcf, User-agent=DAVdroid/2.0-ose (2018/08/03; dav4android; okhttp/3.11.0) Android/8.0.0
[Fri Aug 24 09:29:18.158045 2018] [php7:notice] [pid 10198] [client ::1:55669] PHP Warning: imagecreatefromstring(): Couldn't create GD Image Stream out of Data in /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php on line 1080
[Fri Aug 24 09:29:18.158053 2018] [php7:notice] [pid 10198] [client ::1:55669] #1 /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php(1080): imagecreatefromstring('\\xFF\\xD8\\xFF\\xE0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00...')
[Fri Aug 24 09:29:18.158059 2018] [php7:notice] [pid 10198] [client ::1:55669] #2 /opt/local/apache2/htdocs/egroupware/api/src/Contacts.php(922): EGroupware\\Api\\Contacts::resize_photo('\\xFF\\xD8\\xFF\\xE0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00...')
[Fri Aug 24 09:29:18.158064 2018] [php7:notice] [pid 10198] [client ::1:55669] #3 /opt/local/apache2/htdocs/egroupware/addressbook/inc/class.addressbook_groupdav.inc.php(675): EGroupware\\Api\\Contacts->save(Array)
[Fri Aug 24 09:29:18.158070 2018] [php7:notice] [pid 10198] [client ::1:55669] #4 /opt/local/apache2/htdocs/egroupware/api/src/CalDAV.php(1741): addressbook_groupdav->put(Array, '7b83a8eb-5efd-4...', 5, '/ralf')
[Fri Aug 24 09:29:18.158075 2018] [php7:notice] [pid 10198] [client ::1:55669] #5 /opt/local/apache2/htdocs/egroupware/api/src/WebDAV/Server.php(1840): EGroupware\\Api\\CalDAV->PUT(Array)
[Fri Aug 24 09:29:18.158086 2018] [php7:notice] [pid 10198] [client ::1:55669] #6 /opt/local/apache2/htdocs/egroupware/api/src/WebDAV/Server.php(306): HTTP_WebDAV_Server->http_PUT()
[Fri Aug 24 09:29:18.158092 2018] [php7:notice] [pid 10198] [client ::1:55669] #7 /opt/local/apache2/htdocs/egroupware/api/src/CalDAV.php(2053): HTTP_WebDAV_Server->ServeRequest(NULL)
[Fri Aug 24 09:29:18.158097 2018] [php7:notice] [pid 10198] [client ::1:55669] #8 /opt/local/apache2/htdocs/egroupware/groupdav.php(52): EGroupware\\Api\\CalDAV->ServeRequest()
[Fri Aug 24 09:29:18.158103 2018] [php7:notice] [pid 10198] [client ::1:55669] #9 {main}
[Fri Aug 24 09:29:18.158111 2018] [php7:notice] [pid 10198] [client ::1:55669] # Instance=boulder.egroupware.org, User=ralf, Request=PUT http://boulder.egroupware.org/egroupware/groupdav.php/ralf/addressbook/7b83a8eb-5efd-46c4-bb44-df8ceb20c80b.vcf, User-agent=DAVdroid/2.0-ose (2018/08/03; dav4android; okhttp/3.11.0) Android/8.0.0
[Fri Aug 24 09:29:18.158123 2018] [php7:notice] [pid 10198] [client ::1:55669] EGroupware\\Api\\Contacts::resize_photo() invalid image!
Sprich irgendwas ist an dem Bild kaputt oder wird von der JPEG Library die die GD Erweiterung von PHP nutzt nicht verstanden.
Mir ist auch aufgefallen, das die Zeile 347 deines Logs länger ist als alle andern, das ist sehr merkwürdig.
Versuch mal ein anderes Bild.
Ralf