Again, many thanks again for looking deeper into this problem. I, too, tried to reproduce the problem with my own images but could not. So I tend to agree that this is a probably user specific problem. So far, only this one user has reported the problem.
Over the weekend, I tried to optimize the code by using code from another similar set of pages and it seems to me that it is now working. At least with my own image files. Perhaps, that is why you did not experience any issues if you only looked today.
One thing I did notice was that the File Manipulation "Upload Files" server behavior generated a slightly different code for setting the session id for the file name than an earlier earlier version of Data Bridge.
'FileName' => "thumb_\". session_id() .\"",
whereas the backslash escapes the quotes
'FileName' => "thumb_". session_id() ."",
I am using the older previous code now on the working pages. I don't know if this is relevant.