Hi, @crossi72,
I just downloaded your plugin for a client today and found the same error. In my particular instance I was trying to update a media item that was uploaded some time ago.
The issue is that the filepath is incorrect when trying to upload.
On line 501 of class-media-rename you use wp_upload_dir() to get the file pat array, then on line 514 use this array to return the path. However this returns the current path to uploads directory, which can change every month. Currently for me this is /wp-content/uploads/2020/09/ which is not the directory that the media item is in. Obviously this will work for items that have just been uploaded and renamed, but not for older items.
A fix I have found is to replace line 514 with $filepath = str_replace(basename($filename), '', $filename);
which gives the actual file_path.
You would have to update $subfolder
a well but I haven’t looked that far yet I’m afraid.