SyntaxError: JSON.parse: unexpected character

Hello,
I have installed Frontend File Manager Plugin on my wordpress site : https://amstrad.eu
when I try to upload a file, it seems to do the job, but a the end, the browser (firefox) keeps waiting and I noticed
this error in the javascript console :

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data wpfm-file.js:255:33
filecomplete https://amstrad.eu/wp-content/plugins/nmedia-user-file-uploader/js/wpfm-file.js?ver=5.2.2:255
complete https://amstrad.eu/wp-content/plugins/nmedia-user-file-uploader/js/fileapi/dist/FileAPI.min.js?ver=5.2.2:2
end https://amstrad.eu/wp-content/plugins/nmedia-user-file-uploader/js/fileapi/dist/FileAPI.min.js?ver=5.2.2:2
onreadystatechange https://amstrad.eu/wp-content/plugins/nmedia-user-file-uploader/js/fileapi/dist/FileAPI.min.js?ver=5.2.2:2

I have noticed that the files I selected were indeed uploaded on our server in /wp-content/uploads/user_uploads/<user name>, but
anyhow I can’t see them in Frontend File Manager Plugin. It keeps saying that I have 0 files uploaded.

Can you explain me, what to do to correct this, or maybe if it’s a bug correct it in the next update ?

Yours sincerely