Probably the first thing to mention is that every time I save a project file I get the "not responding" message in the title bar for about 10 seconds - this may or may not be significant.

I processed a file (DNG) in Studio and created a .TSP file for future processing.  When I tried to open this .TSP file, from within Studio, I got the error message: "Error saving or opening project file." and the file was completely removed from my PC.  I created another version of the .TSP, and while I got the same error message on opening it, this time it wasn't deleted.  I then tried some older .TSP files I have and they opened without a problem.

I opened a ticket with yourselves, and then mentioned the problem to a friend of mine who offered to try the file.  At this point neither version of Studio would even launch. My friend opened the file with no problem and after processing it further returned it to me.  

I then rebooted my PC at which point both versions of Studio started to work again. but I still could not open either my or my friend's .TSP files, BUT I found that when I double clicked on one in Windows Explorer it opened in Studio (all previous attempts were using File, Open within Studio).  This also worked with the other file, and once I'd opened them both this way I was subsequently able to open them both from within Studio.  All well and good, though not exactly how things should be.

However, a bit later I moved my friend's .TSP to a different location (and renamed it) and got the now familiar error message when trying to open it and the file was once again deleted from my PC.

Next I created a copy of my version of the .TSP, double clicked on it, got the error message and the file was deleted. I created another copy, double clicked on it and got the error message, but the file remained, albeit unopenable.

Next, I shut down and retarted my PC and on the next attempt to open the copy it was deleted (usual error message of course).

Opened original .TSP and saved settings as an effect, applied to a JPEG, and new TSP opens without a problem.  Same with a DNG and a raw (RW2) file.

My second version (of which I made the now deleted copy) is still there and still opens normally.

LATEST UPDATE:

I now don't get the "not responding" message and things appear to be working normally in terms of creating and then opening a .TSP.  However, if I make a copy in Windows, opening it produces the error message and it's deleted.  If I then recreate the copy with the same name, opening it gets the error message but it isn't deleted.