Nabble removed Mailing-list integration from the Forum.
This killed the Forum. This is now an ARCHIVE.
It is likely Nabble will shutdown in the future.
So basically the Forum is now out of date, we are looking into migrating the history.
OpenSCAD support is now provided by the Mailing List.
See the first pinned post here for instructions on subscribing to the mailing list.
Problem with a twisted prism