Thumbnail not writeable?

Hi,

A permission of 755 might not be enough depending on who the owner is.
For example, if you copied the upload folder content from a server to another the linux user owner of the folders and files might be the FTP user you used to connect to the FTP server to upload the files and folders.
And your website could use a specific user linked to the web server ( apache / nginx ).
In such a case, the users being different, it's not the first number which applied but the second or the third. And a "5" won't be enough to write the folders / files, and thus you can get that error.
The solution in such a case is to change the owner of the folders / files so that it is the same as the user used by the web server when processing the pages. That way, it will be able to use the "7" permissions, while preventing other users of the server from writing files / folders.
If you''re not familiar with all this, it's probably something you want to check with your hosting provider support.