I've just donwloaded the new version, uninstalled the old, installed the new, and the game does not open anymore.
Not only that, but my ".t-engine" folder (i'm on linux mint) has dissapeared.
I've informed the creator, but thought it might be a problem from GOG (as nobody using the other installers seems to have this problem), and I'm telling you here too, just in case...

Ok, I can see is something related with the libz.so.1
The game asks for it, and while is there and I have the libraries installed, it says it's not there...

Did you install the game accidentally into a subfolder instead of overwriting?

Did you remember to install the DLC too if you've got it?

I installed in the correct folder, and installed the DLC.
I've gone back to the previous version, and everything works fine. I've uninstalled it, and then installed again, and nothing. I've tried another folder, and nothing. So, for now i've gone back to 1.5.10...
Oddly, in a different computer the same donwload works like a charm,..

Here's a hunch, your install location. Is it Program Files (either of them?)

No, I'm using linux mint. I'm thinking i'ts a library problem with the version 18 of linux mint, as all the times i've installed it in a Linux mint 18 has failed, but in Linux Mint 19 works...

Yeah, dunno how to help you with that. I'm using Fedora 31.

I had possibly related problem. When I started the (updated) game, it failed because it did not found t-engine.
It turned out that start.sh was looking for t-engine in game/ folder, while it was in game/game/ folder; similarly for lib32/ and lib64/ folders. So I changed appropriate lines in start.sh script (see below) and game seems to run fine.

This is how the appropriate lines in start.sh look after change:

local bin_path32="$CURRENT_DIR/game/game/t-engine4-linux32"
local bin_path64="$CURRENT_DIR/game/game/t-engine4-linux64"
local lib_path32="$CURRENT_DIR/game/lib32"
local lib_path64="$CURRENT_DIR/game/lib64"