Hi Sylvio,
Well, that looks exactly like a regression : the Lab color curves does work on 1.6.0.3/Windows, but not on 1.6.1.0_beta/Windows.
Today, I've already tried to reproduce this bug on Linux, but with no luck (but you say all is working on Linux too).
I'll try to reproduce the bug on Windows, on Monday. I'm just hoping this will crash, otherwise I won't have any ways to fix this.
Could you try also with the 'Lch' colorspace and 'YCbCr' (maybe the first could crash too, not the second).
Maybe you could help me a little bit. Could you do these steps :
1. Open the G'MIC plug-in, set 'Output mode' to 'Debug mode(logfile)'.
2. Run the interactive color curves in Lab colorspace. Do the thing to make it crash. Be patient, as the 'debug' mode is much slower, so it may not be as responsive as in 'normal' mode.
3. Go to the %TEMP% folder (on XP : C:\Document and Settings\user\AppData\Temp\) and grab the 'gmic_log' file there.
4. Paste in somewhere (on pastebin for instance), and share the link

This could help me to locate where the crash occurs more precisely.
Thanks again.