Here's some temporary fixes:
If you're copying the text from somewhere on Toyhouse, try copying the source code and pasting that instead of the formatted text.
Otherwise, right-click and hit "Paste without formatting" or "Paste as plain text". You'll have to re-add any existing formatting but that'll get rid of all the weird issues that pop up.
We've never had issues with copy/pasting so we can't be much more help, sorry ^^