Hi Francisco,
This is a confirmed regression in PixInsight 1.5.5 for Windows (the Linux and Mac OS X versions don't have this problem).
After a hard morning of fight against thousands of intricate source code lines, I finally have managed to find where this bug is (was), and have fixed it. Of course the bug fix will be included in PixInsight 1.5.6, which is due for release before next Monday.
Thank you for reporting, and sorry for the inconveniences!