Try disabling NeatlineFeatures...it's related to https://github.com/scholarslab/NeatlineFeatures/issues/81.
Thanks but I did that (even uninstalled NeatlineFeatures) and it still does not save. Is there anything else I could try?
Could you tell me what operating system and browser you're using? We'll need to get more information, and once I have that, I can tell you how to get the extra information that might help us.
I'm using Google Chrome 40.0.2214.115 on Mac OS X 10.9.2. Also tried it with Chrome on Ubuntu 14.04 with the same result.
Do you have mod_security installed by any chance? I reread your message again and 501 errors are generally from server settings blocking certain types of actions (like PUT in this case).
Disabling mod_security on the server did the trick, I can now save records in Neatline. Thanks!