Multi File Attachment not working

Since updating the website including all plugins, OS, Theme etc. users are now unable to load multiple files. This has been working for several years with no issues.

I’ve deactivated anything that might be bothering the plugin, I’ve also deleted and reinstalled the plugin.

Only when I make the form allow a single file to be uploaded – then it works. Example:
[file file-737 limit:1024kb filetypes:|png|jpg|jpeg]
(on the mail said under file attachments, its set to this code: [file-737])

But when I switch it back to multi file:
[multifile multifile-706 limit:1024kb filetypes:|png|jpg|jpeg]
(on the mail said under file attachments, its set to this code: [multifile-706]).
It does not work.

Why?

The page I need help with: [log in to see the link]