I recently updated the forum to version 1.1.20, which is what caused this - and only for certain browsers (versions of Firefox, largely). Fortunately, other users also reported the bug and one of the programmers was able to identify the cause, and offer a fix. I made the manual changes to the source code required.
Preview is now working for me. Please let me know if this clears it up for you.
Yes it has. Thank you!