I'm also having this problem with hactoolgui. I tried manualy entering the key on the keys.dat file with notepad++ and it still did not recognize it. It also corrupted the other keys I had saved.
Let me know if there are any bugs or features you guys want, I defaulted the save path to the same location as the input file, if you guys want it in the hactool directory (or want to choose the path) let me know
Any coding suggestions/improvements let me know as well (or submit a pull request!) and I'll include it, I want it to take care of the needs of the people