It seems that this is caused by the locale setting being one that uses a comma as the decimal separator instead of a dot. I don't really know why it would make any difference, since my code is not even looking at that, but maybe Qt or jsoncpp are looking at it. I will try to find out the cause.
In the meantime, you can add this to the shell script that starts RUBE, to make the program run with the en_US locale:
export LANG=en_US
The shell script is the small file called 'rube' that was in the download, which you can edit with a text editor (the larger file 'rube.bin' is the program itself).
The LANG variable will set any other non-specified language variables, so that is usually enough, but if you have specifically set your individual locale variables to different things, you might also need to add this to be safe:
export LC_NUMERIC=en_US
I will add this to the script for future downloads.