Uploading images on the frontend

I’m currently developing a subscription based site, where users with the user role ‘client’ must be able to upload files. I’m handeling the rendering of the form via the acf_form() function.

Via the ‘Members’ plugin I have set the capabilities for ‘clients’ to ‘upload_files’, nonetheless I always receive the error ‘Error occurred while uploading. Please try again later.’

This issue has nothing to do with server limitations or whatsoever, since when I change the user role of the user to ‘editor’, it all works fine. So I am wondering what capability is this user role missing to properly upload files via the wp media uploader?