Invalid HTML error when publishing new stories. Please help.

I am trying to post a story with images on one of my threads, but keep getting an error message come up saying: 'This content is not published. Invalid HTML. Please remove this before posting. Lib- sim.' (no idea what the last bit means). 

The story is pasted in plain text from a Word doc and have tried uploading each image via the option in the bottom bar (From Computer) instead of copying and pasting but it still won't let me publish the story. There is no HTML content used in the story, it's all from my computer, so am not sure why I keep getting a message about invalid HTML content.

I am wondering if it's the length that's causing issues, as when I do shorter posts with one or two images, it works perfectly fine.

I notice there doesn't seem to be a spoiler option on the bar for new posts/replies after the main introductory post so I can't get the long post into a spoiler.  

Has anyone else had the same issue when trying to post stories (can't find related topics in Search)? And if so, have you managed to solve it?