I have several sites all running Wordpress. One site was hacked but after reinstalling a clean backup copy it would not recognise any of the themes. Cut a long story short, I deleted all files and database tables and installed a completely new fresh wordpress, both database and files. But it still would not connect to the themes. After checking and trying everything I could think of I finally tried it in different browsers - and it was working! So it appears that it is just Firefox that it won't work in. I will say that all my other sites work fine in Firefox, and it also worked in Firefox on the copy of the site I have on my local. Tried to upload image to show themes in admin section being shown as just empty image boxes, but your upload image here just kept whirring round and round!