If you could not see the thumbnails, then your problem was related to /cache folder permission issues. Use the free Admin Tools from akeebabackup.com to fix your site's read/write permissions. If you still face this issue, then your server is not properly setup and your file/folder permissions have the wrong "owner". In that case, you need to ask your hosting company to fix the "owner" for files and folders on your entire hosting account.

If they refuse, you should consider changing hosting company. We highly recommend companies like: www.eurovps.com or www.rochenhost.com