I’ve had this happen to me when we were on 6.5.4, but not since the move to 8.0.1.
Unfortunately, I never managed to find a way to recover the form, and subsequently lost about half a day’s work! There are quite a lot of posts on the issue in the 6 & 7 forum.
The error usually means the form is corrupt and has somehow lost the $body, which contains pretty much all of the useful information for the form, so without that there’s no way to get it back. The only hope is probably to try to find a fairly recent replica so you don’t have to do everything again.
There’s an item in the design note of a type that the Designer code doesn’t recognize. If you can figure out which item it is (from comparing the design element properties with those of another form), then you can delete the item or replace its value by using a toolbar icon formula from the view of design elements. If it’s the $Body item, however, that’s most of your form so you may be out of luck.
Subject: ugh “Not a Form” generic undescriptive error…
Just ran into this error on a 8.02 designer client with a Standard install. No warning, I try to open the form and get the “not a form” error. I try to create a document with that form in the client and it crashes notes. I can look at the design properties but nothing else. I can copy and paste the design element too. I did manage to export the form design with the dxl export and may be able to recover some of the LS code. Fixup didn’t do anything. Time to reinstall my Snapper.
I had the same problem with one of my Forms.The only thing that helped me to get some code from my Form is the using menu action Tools → Edit with DXL (Lotus Notes v.8.5.3).