Yeah the CASC format changed between 1.32.9 and 1.32.10 in one of those "do nothing" patches from Blizzard. The new format does not include metadata to inform an application that it is in a new format, it's just different. @Dr Super Good
maintains the JCASC library that is used by Retera Model STudio so maybe he will think of something later to allow his JCASC library to load from both the old and new CASC.
But, until that happens, you are stuck with either
(1) using an old version of Retera Model Studio
(2) compiling your own Retera Model Studio where you change that 1 line of code that's different in the CASC parser back to 1.31 mode. The difference is whether we skip over a binary byte deep in the TVFS definitions or something like that, but @Dr Super Good
wrote the JCASC parser that I use, so he knows more.
??? I heard a rumor that maybe
your problem might be fixed in the "twilac Retera Model Studio 0.05 version" that others are working on now that I was being lazy and not working on Retera Model Studio myself. I recall reading something about that, where maybe twilac wrote some crazy code to guess at the contents of the CASC and then based on whether the binary skip was present or not it would guess between the versions. I don't really bother with that. When I want to deal with 1.31 assets, I have a folder that I use where I had previously extracted all the game's contents with CascView, so I just add it as a "Folder" data source in the menus. Speaking of which, actually, let me add point 4 below:
(4) Edit: maybe just use CascView to extract all the contents of 1.31 to a folder, then use "Add Folder" in the Retera Model Studio data setup
Alternatively, since 1.31 is just the Reforged team's work before they were required to deface it with 1.32 or whatever, you could consider to either use the full version of their work (1.32) which will prohibit you from running old versions so you don't have this problem, OR you could use the good old version of Warcraft III from before the Activision merger. Retera Model Studio should generally support any of the patches of Warcraft III from before the Blizzard/Activision merger, since they all use the consistent MPQ format and don't have issues like this.
(As far as I know, 1.23 was the first patch after the Blizzard/Activision merger, so most all the patches after that are either pointless or prep for Reforged)