Error 404 – Not Found, when trying to save post

I know there have been discussions on this topic in recent years but none of the suggested ‘fixes’ actually work for me. I have been posting to my site for 1 1/2 years with no problems until now. I have talked to my host. I have disabled plug ins. I have tried this forum. It’s to the point now where I cannot post anymore. I’m at a standstill and I can’t find a workaround.

On my current post that I’m working on, I was able to save up to page 4, then the next sentence apparently caused the Error 404. I tried entering a random sentence instead of the ‘offending’ sentence I had typed. No problem saving the random sentence. I tried pasting the offending sentence into the post at a different location than I wanted it and it saved fine. But if I put the sentence in the location I need it, I cannot save the post. I have less than 1000 words in the post. The auto save draft function also fails to work when this problem is active.

Surely someone has come up with a resolution to this problem by now. I would love to be able to get back to work on my website. I would be eternally grateful to anyone who has the answer to this bizarre behavior by WordPress. Thanks in advance!
Dennis