It indicates too many files in one diretory direct(=except subdir's files), so it can't allocate the area for file attributes.
The most easy way is changing FastCopy(x86) version -> FastCopy(x64) version.
But if your OS is 32bit, try this.
FastCopy2.ini
[main]
max_attrsize = 268435456; (=256MB or 402653184=384MB)
32bit OS user address space is only 2GB and it is used for stack/heap/share memory.
If you change max_attrsize, I suggest not to use big number for main buffer size.
Otherwise, you get allocating error for main buffer.