I've seen this several times when installing or updating - it always complains that it cannot write one of the font (.ttf) files.
I am using the standard directory structure for Portable Apps - so, given the standard .paf. installer I wouldnt expect a problem with path lengths.
There is miles of space on the USB drive (its 32Gb) and the drive is OK and passes a drive check.
If I copy the structure from another drive it copies OK (including the file in question) and runs OK.
BTW I love the package.