THE PROBLEM:

Out of nowhere this error appeared:
Failed to save Bilingual as : Could not find part of the path
I noticed that the error occurred when I tried to save the project.
The invalid path referred to in the error message was in my case:
C:\Users\joaom\AppData\Local\Temp\SDLTempFileRegen\sdlxliff_37b351b0-57de-4c51-b3f2-b7f8c9cde33f.tmp

THE SOLUTION:

I went to the directory path:
C:\Users\joaom\AppData\Local\Temp\
And recreated the folder "SDLTempFileRegen" which was not present.
After that I was able to save again without the error occurring.

Hope that helps someone. =)