As a workaround, follow this workflow:
1. Cap the top with a planar surface
2. Knit the 2 surface bodies and for a solid
3. Shell the solid with .5 mm
You will get this warning:
The Thickness value is greater than the Minimum radius of Curvature. The shell may succeed, but could cause undesirable results, such as bad geometry. To find the Minimum radius of Curvature, use Tools, Check.
That being said, the end result might be close enough to what you need:
BTW, there si an interesting warning on the shell feature in the tree:
OMG thank you so much! Finally got it to work!
It's cool that Alin solved your problem by using the Shell command. We often end up using Surfacing commands to work our way around failures of the Shell command and now the Shell command has the crowing rights. On the other hand, the warnings that the command gives you are worth listening to and thinking about, even if you decide to forge ahead. (Which I invariably do.)
You can often track down the problem areas in your part by using a smaller value for the thicken, doing a binary search to nail down the maximum thickness that will work. Visual examination of the maximum thickness part will often show where are an edge or surface is about to disappear. Sometimes you might find that a thicker part works, in which case you can do a similar binary search for the thinnest part that will build. Another approach is offset a surface from groups of faces to see if certain boundaries are causing the difficulty.
You might also want to run the stringent Check tool on your part to make sure that it is OK. Even a part that checks good may go bad later as you add more features. You might have to return to the early features to "fix" them so that the later features will work.