shmerl: I didn't try it yet since I don't have other partitions now. I can give it a go on a small USB drive for a test.
jonaswolz: OK. Alternatively, for a test, you could create a small file system in a file using a loop device...
Ah, that's a great idea, thanks. I did just that and it worked, which confirms your findings above.
Here is what I did (assuming ~/tmp/image was already created before):
dd if=/dev/zero of=/tmp/loop.img bs=1024K count=800
mkfs.xfs /tmp/loop.img
sudo mount /tmp/loop.img ~/tmp/image -o loop
Then I copied clarc game into ~/tmp/image (with my user's permissions) and launched it from there, and it worked!
I added the test result to the forum bug report, but Unity developers didn't answer anything so far.