Not sure if it's the same issue, but I have trouble with the tags at the bottom "autofilling" with a bunch of the words I use when I click the "preview post".
I have to make sure to scroll down to delete all those, otherwise it won't let me post because there are too many tags!
This picture is when I do a "preview post" just to show an example, in case that's what you're running into also!
You're probably not doing anything to create the tags-- they just pop up on their own for me, anyway.
When you're ready to post something, you can just go down to the box at the bottom with the tags and delete them all. After that you should be able to post whatever length of writing you would like!