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.