I have the same problem but the difference is i have the google form embed in iframe tags for over a year. then suddenly this morning when i check my website, the embedded google form in pages stop working and only show a blank page.
The solutions:
1. First check whether the iframe tags are working correctly: http://www.w3schools.com/html/tryit.asp?filename=tryhtml_basic
sim
Simply replace your iframe tags in the body and submit the codes. If it is working then all is good.
2. Search from your plugins page: WordPress Google Form or download from here: http://wordpress.org/extend/plugins/wpgform/
simply follow the instructions and all working perfectly in wordpress.
Good luck!
I also receive the same error. It uploaded correctly but no thumbnails shown after uploading and from the dashboard, nothing shown but when I checked from the FTP: uploads/user_uploads/admin there files are all there.
