After a lot of searching, I found a hint at name, that active sync supports only 48kb of images in contacts.
I located the code in /usr/share/kolab-syncroton/lib/kolab_sync_data_contacts.php in line 170 and changed the content from:
// ActiveSync limits photo size to 48KB (of base64 encoded string) if (strlen($value) * 1.33 > 48 * 1024) { continue 2; }
to
// ActiveSync limits photo size to 48KB (of base64 encoded string) if (strlen($value) * 1.33 > 128 * 1024) { continue 2; }
And it works fine for me. So please can someone check my solution and apply it to the master code branch?
Best regards.