Andrea,
I noticed that in .../bin/ffmpeg/plugin.opts (and also in .../ffmpeg/plugin.opts, of course) the "new in 5.0" and "new in 5.1" sections are missing.
"My bad" - I used the file you sent previously but put the new ones in alphabetical order at the top. I hope that is OK as I was using it to verify the plugins.txt file and found it much easier. And then left the broken ones on the bottom so that maybe someday it will be possible for a developer to make some code changes to get them working also. Could you verify "expanders.txt" in the main cinelerra-5.1 directory because I added to FF_Color_Correction, the new ffmpeg plugins of F_grayworld, F_hsvhold, F_hsvkey. To Artistic, I added F_pixelize and to Test, I added F_blockdetect, F_blurdetect, F_colorchart, and F_colorspectrum. I will change anything you suggest or maybe none of them should be added?