I don’t think any of these plugins are causing that error and I’m still suspecting the database. Investigating this issue will require having access to the server and the database. I’m afraid this goes beyond the free support service I offer on WorP’Dev, but I can certainly work on it if you are willing to book me for a few hours. I’m a results based guy, so you don’t pay if the problem is not fixed.
Let me know if you want me to look further into this for you.