The "update" command refuses to overwrite an existing ".old" backup of the parameter file:
# just copy&paste to reproduce
set print "test1"
update "test1" # "test1" is copied to "test1.old" and afterwards updated
update "test1" # throws following error:
The backup file test1.old already exists and will not be overwritten. error during fit
This is in both latest windows testing binaries (465 and 50alpha, the later without the "error during fit"), but i think the problem is somewhat older.
Also gp465 still (see https://sourceforge.net/p/gnuplot/bugs/1156/?limit=10&page=1#92ed ) throws misleading error messages around "update", reported file names are incorrect and the "error during fit" thing shown above is nonsense.