Ok, I found the reason. My old nodes are all 32-bit (we run fog on recycled hardware basically), my new node is 64-bit. In the freespace.php a recent change (somewhere within the last few months) made calculating $hdtotal and $hdused use intval(). This made them integers which is not a big deal on 64-bit systems in PHP but in 32-bit systems the harddrive space in bits is higher than the maximum size of an integer. If I restore the old freespace.php file it works fine.
I don’t know a lot about coding in PHP but is there a way to cast the result of the calculation to a double or long for 32-bit compatibility?