Error 500 – Internal server error.

When I upload a file to the WP Media Library Folders plugin, most of the time I get a popup error with HTML code, where I can read:

“Sorry it didn’t work. Please try again later.
Error 500. Internal server error”

When I connect using FTP, I see the item has been uploaded, it just doesn’t appear in the Library. It creates chaos as I don’t know if I’ve uploaded an item in the past or not.

WordPress 4.9.8 –
File: PNG 162 bytes.

Thanks for any help.