Intermittent 404 errors when previewing

I’ve been using the Total theme for several projects and support has been phenomenal. I had been using Custom Post Types UI for a few projects and began testing the Post Types Unlimited plugin as a possible replacement for that. On several sites, some with and some without Total set as the theme, when trying to preview posts created in a CPT using PTU, I intermittently get a 404 error. Not sure what is happening but it is keeping me from moving forward with it as a replacement for CPT UI.

I could create a video to show what is happening, but essentially, when previewing a post, I sometimes get the preview as expected and the URL is clean, like this (from a local test site):

http://localhost:81/wordpress/self-help/some-kind-of-kb-article/?preview=true

When I get a 404 instead, the URL contains much more information and looks like this:

http://localhost:81/wordpress/self-help/some-kind-of-kb-article/?preview_id=75&preview_nonce=27ffd3ad0a&post_format=standard&_thumbnail_id=-1

I’m still a bit of a babe in the woods with some things WordPress does so this could be something I am doing. I’ve experienced it in several environments (from IIS to WAMP) with different versions of WP running and all seem to have this problem.

Thanks.