No one has a clue? I'll file a bug to GOG tech support and developers then.

If you run it under strace, you should be able to figure out where it's looking for said data.

I can't try that myself because CLARC is still on my wishlist, waiting for a discount deep enough to make up for the fact that I own roughly 2/3rds of GOG's catalog and have played only a tiny fraction of it.

Something weird is going on with it.

Running strace -s 64 ./CLARC_LINUX.x86

produces (showing just the last part):

write(1, "Set current directory to /opt/games/clarc/game\n", 47Set current directory to /opt/games/clarc/game
) = 47
write(1, "Found path: /opt/games/clarc/game/CLARC_LINUX.x86\n", 50Found path: /opt/games/clarc/game/CLARC_LINUX.x86
) = 50
readlink("/proc/self/exe", "/opt/games/clarc/game/CLARC_LINUX.x86", 1023) = 37
stat64("/opt/games/clarc/game/CLARC_LINUX_Data/mainData", {st_mode=S_IFREG|0644, st_size=56080, ...}) = 0
readlink("/proc/self/exe", "/opt/games/clarc/game/CLARC_LINUX.x86", 1023) = 37
stat64("/opt/games/clarc/game/Data/mainData", 0xfffc28e0) = -1 ENOENT (No such file or directory)
write(1, "There is no data folder\n", 24There is no data folder
) = 24
munmap(0xf48e4000, 1052672) = 0
getpid() = 16891
exit_group(1) = ?
+++ exited with 1 +++

/opt/games/clarc/game/CLARC_LINUX_Data/mainData indeed exists. However /opt/games/clarc/game/Data directory doesn't exist (so no files in it can exist either).

OK, I copied CLARC_LINUX_Data to Data (having both in the same parent directory).

Now it's even more weird:

write(1, "Set current directory to /opt/games/clarc/game\n", 47Set current directory to /opt/games/clarc/game
) = 47
write(1, "Found path: /opt/games/clarc/game/CLARC_LINUX.x86\n", 50Found path: /opt/games/clarc/game/CLARC_LINUX.x86
) = 50
readlink("/proc/self/exe", "/opt/games/clarc/game/CLARC_LINUX.x86", 1023) = 37
stat64("/opt/games/clarc/game/CLARC_LINUX_Data/mainData", {st_mode=S_IFREG|0644, st_size=56080, ...}) = 0
readlink("/proc/self/exe", "/opt/games/clarc/game/CLARC_LINUX.x86", 1023) = 37
stat64("/opt/games/clarc/game/Data/mainData", {st_mode=S_IFREG|0644, st_size=56080, ...}) = 0
write(1, "There is no data folder\n", 24There is no data folder
) = 24
munmap(0xf49b4000, 1052672) = 0
getpid() = 16969
exit_group(1) = ?
+++ exited with 1 +++

It found /opt/games/clarc/game/Data/mainData now, but still exits with "There is no data folder" message (at least there are no system calls errors in that variant). It's clearly some bug in the game. See above.

Definitely strange. If that's not a full-blown bug, then the error message is catastrophically inaccurate.

Aside from finding someone skilled enough to try to puzzle out what's going on from a gdb backtrace that only has debugging symbols for the open-source parts, I can't think of anything else to do other than wait on GOG support.

I wasn't sure whether I should file a bug report to GOG, since they said they won't support other distros, but it won't hurt trying I guess. I already filed a report to developers, but they didn't answer anything so far.

Have you been able to fix it? If not, maybe it's worth taking a look at the permission and ownership settings of the files and directories.
The tarball works on my box as it is. I suppose you extracted it to /opt? Try the following:
cd to /opt and enter sudo chown -R root:root CLARC (CLARC is the default folder name of the game when it comes from a tarball).
Then please check if the permissions of all the folders including and below CLARC are set at least to rwxr-xr-x
In case it's not set so, please cd to /opt/CLARC (or whatever your game folder) and enter this:
sudo find . -type d -exec sudo chmod 755 {} +
This will set all directories (aka. folders) to rwxr-xr-x

Maybe it helps ... I hope so. ;)

Ownership is fine for me. I tested it in my home directory. It just doesn't work. My filesystem is XFS. It seems to be one of those ports where developers just made a Unity export for Linux version and don't care to fix actual bugs.

No response from developers. It seems they don't care about bug reports.

One quick question: do you have Mono installed to your system? The game seems to use Mono.

I don't understand what you wrote about the mainData folder. I also used the tarball to install the game, but there is no mainData folder. The game works for me, though.

You have tried installing the game to /opt and to your home directory and it failed both times with the same error?

I would bet on a missing dependency or a dependency the game can not resolve (if that's the case and we manage to find it, symlinks can help ...) I suppose you have already checked for the dependencies.

I don't have separate Mono installed. The game ships its own Mono libraries however, so they are supposed to work, but the end result is failing with those rather weird errors.

Exactly, it comes with one library but not with the Mono runtime environment as far as I can see. Try installing Mono runtime and see whether that changes the issue.

I had this problem with another game with the Unity engine in Linux (Shadowrun Returns).

It seems to be related to the partition size of the game folder:

When the folder was on a (XFS) partition with 2,6TB size and 1,8TB free, it failed with "There is no data folder".

When I moved the folder to a smaller partition (also XFS, 200GB size with 130GB free), it worked flawlessly.

I don't know why this is so, however...
Nevertheless, I hope this workaround may help other users.

Yeah, my situation is similar (XFS, and big 1+ TB partition), so it looks like you are on to something. Did you try reporting it to Unity developers? If not, I'll file a bug.

I have not filed a bug report so far, so please feel free to do so...

Just out of curiosity:
Have you already tried if it works for you too when you put your game on a smaller partition?