I'm glad that you are up and running again. Do you recall what file was locked? That's not something we can control but I would like to find out how it happened.
Fyi I've been having locked file issues ever since the 2.5 Opt-In first went up, and still am. I don't know which files are locked b/c nothing tells me. It appears to be random and different every time, and doesn't happen at first, only after I've been modding for 15 minutes to 2 hours. But when I go to save an edited XML file in my text editor for example, it will complain about not running in Administrator Mode all of a sudden, after having been saving fine for a while before that. If I then relaunch the text editor in Administrator Mode and try the save again, Windows says there was an unexpected error, and nothing else.
Interestingly, I found that restarting my PC does NOT help at all - whichever file it was, it still won't save. BUT, if I log out of Windows and then back in (this is Windows 10 Pro 64-bit, btw), it's back to normal again, at least for a while. Since relogging is significantly faster than rebooting, I found this very helpful to know lol.
But yes when I hadn't tried saving any text files recently, but was copying files around between different versions of GC3 and/or different versions of my mod for example, the game would occasionally load fine but crash when attempting to start a new test game. I found that relogging out/back-into Windows also fixed this whenever it happened, so it seems like locked files were to blame again, but I don't know which ones. It seems like it can be any of them in the GC3 folder, at random, but I don't know for sure.