I normally keep my spell-check turned off, then run it, so this bug may not have been spotted yet. After I get to the last flagged word, the okay box pops up, but it’s grayed out. Essentially it’s frozen, and the only way to return to write is to bring up the Task Manager and end the program from there, then reopen.
Thanks, someone else did report this and we have fixed it for the next update. In some cases it’s possible to close the window via the “X” button in the upper right, but I know one other person (running XP) wasn’t able to do so and had to force quit as you’re describing. The “OK” button will be working properly in the next update.