So I found this on both Windows & Linux + Opera, if you use an iframe html tag within a post you can't post or even edit the post once you leave the page.
I also have a way to fix the issue for yourself. It doesn't allow you to post the iframe html tag however you will be able to upload a post again.
Go to Settings>Privacy & Security> Cookies
Search for cookies
- Click 'All Cookies and Site Data'
- Search for 'Steemit'
- clear the data for steemit
WARNING: this will log you out of steemit and delete all steemit related cookies.
- log back in and start Steeming again.
If you find a bug with steemit you can report here: https://github.com/steemit/condenser/issues
This post also gave a work around to solve the issue.