I'm having the same problem with 4.91.  What's happening is that the Upload Skin function is creating two directory levels instead of one.  For example, if you upload a skin called NEWSkin, it creates the following:

           Skins
                   NEWSkin
                                 NEWSkin

And the actual files are getting loaded into the 2nd level directory so when you select that skin (the name is there because the folder was created) it can't find the files because it's looking at the 1st level instead of the 2nd.

As a workaround, after uploading I went into my site using FTP, moved the files from the 2nd level to the first directory with the same name and then deleted the 2nd level folder (now empty).  Everything worked fine then, but it is a pain.  Not sure if it's new to 4.91 or I'm doing something wrong.  I tried a variety of ways of creating the ZIP files to no avail.

Hope this makes sense...let me know if you have any questions.