I was getting the same error.
I checked the response in Firebug. It said that ".../assets/image-gallery/New-ImageGalleryPage-2" did not exist. I checked assets/image-gallery and it had a folder named "New-ImageGalleryPage-3". I renamed it to "New-ImageGalleryPage-2", but every time I tried to save the page, that folder would be renamed to "New-ImageGalleryPage-3" and then the error would appear again.
I just blew away all my Image Gallery Pages and all the folders in "assets/image-gallery". When I created a new Image Gallery Page, the problem seemed to correct itself.
I am getting the same error here. I installed image_gallery with uploadify instead of SWFupload.
I can create a new gallery page, but as I'm trying to save it I get "Error saving content".
You need to download Uploadify. http://www.leftandmain.com/silverstripe-modules/2010/08/26/uploadify/
SilverStripe tips, tutorials, screencasts and more: http://www.leftandmain.com
Oh but I have. Ran /dev/build too. Removed all previously added imagegallerypages, deleted all previously added folders.
This time I think it was important to remove the folders through the "Files and Images" interface, not just manually from the fs.
Anyway - it helped, I all works now.