start grass from xy location in terminal.
run i.group (select your existing group)
i.target
i.points (already exists in ur case)
i.rectify (without -ca flag)
if the error repeats when run from terminal
you should goto grassdata directory and (ctrl+H ,view hidden files) then u can see .tmp directory you should delete .tmp directory from both xy and projected location mapset folder
delete directory. and start grass again.
Rashad