Hello, Johnny Zhang,
1. yes, I added custom webparts and standard webparts to the site
2. I started with a "Blank Site" and populated it with some standard lists, libraries and webparts (custom and standard)
3. The webpart remains there, once it has been put onto the site. I "touched" every list, library and webpart just once in order not to confuse the SharePoint.
Finally, even after emptying the recycle bin and creating a new site from "Blank Site" template with absolutely nothing on it, it failed again.
Might it be that some settings are faulty?
Thank you for your help.