[Koha] Problem with batch cover image upload

[Mai]

Dear all,
I'm running Koha 16.11 on debian 3.16 and every time I attempt uploading a
.zip file with cover images and datalink.txt I get an error 500.
An error has occurred!*Error 500*This message may have been caused by any
of the following reasons:

   - You made use of an external link to an item that is no longer available
   - You followed an outdated link e.g. from a search engine or a bookmark
   - You tried to access a page that needs authentication
   - An internal link in the client is broken and the page does not exist

What's next?

   - Use top menu bar to navigate to another part of Koha.
   - To report a broken link or any other issue, please contact the Koha
   Administrator

I've checked out Bug 20564
<https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20564> and this
thread
<http://koha.1045719.n5.nabble.com/Bulk-Patron-Image-upload-zip-file-giving-Error-500-td5940031.html>.
When I check my intranet error logs, I even get the same but still, I don't
understand how to fix this. I've also tried it on Koha 18.05 with the same
outcome.
Can anybody help me?
Thank you all in advance.
-- 
Maite Arakaki