I have been getting this for a while now too. I haven't thought much about it. Not sure about an entire fix, but I have been doing the following lately.
Usually I refresh the page after about a minute of waiting for the preview and then immediately click preview, even if the page is still not fully loaded. After that, the page loads with the preview which I am able to see. Sometimes it works and sometimes it doesn't; refresh the page if it doesn't the first time. (I actually had to use this method just now to preview this post.)
You can also try clearing your browser's cache and cookies, but that may not show any difference.
Hopefully trying that quick method (not really a "fix") will help you both be able to preview your posts. Hopefully someone knows of an actual fix to this issue.