sflwa on "Class __PHP_Incomplete_Class has no unserializer"

Given the current randomness of your issue, it's hard for me to come up with some advise. Usually, when one sees an issue, the first step is to reproduce it using a different browser. One that has had its cache completely wiped. Then tested again using the original browser that has also had its cache wiped.

Once the issue is reproducible over a few different browsers -- and if possible computers -- then it's a good idea to rule out any 3rd party incompatibility issues. To do this all one needs to do is temporarily disable ALL of their plugins AND switch back to the default Twenty Sixteen (or one of the Twenty series) theme.

But, given your issue's randomness, I just tried to reproduce the issue using a variety of browsers. All of which had had their cache wiped and could NOT reproduce the issue:

Steps taken:
1. Go to http://tbsmo.com/web-edit/

Result:
Looked over site for any kind of Waing message(s). Nothing found.

Browsers tested:
- Mac Firefox 45.0.2 (debug = false)
- Mac Firefox 45.0.2 (debug = true)
- Mac Chrome 50.0.2661.86 (64-bit) (debug = false)
- Win Edge 25.10586.0.0 (debug = false)
- Win IE 11.212.10586.0 (debug = false)

What were the exact steps you took to reproduce the issue? Is it possible one needs to fill out the form and submit it or something like that in order to reproduce the bug???