Hi I had a recent problem where I could not upload single photos anymore. I would get diverted to an error 406 problem. This is in a similar vein to this issue:
I did all the htaccess changes, which did not work. I even did a fresh install and the error was still there. I concluded this was a minor background config change by the domain hosting company. It was.
After speaking with them, they changed a rule in MOD_SECURITY and it fixed the issue. This message to assist others.