Included to NS3.0 there's a new 'buildafm'-routine (for non-NeXTers: 'buildafm' is a shell script which announces a new font to the computer) at /usr/bin/buildafmdir. The new one only allows to install about 256 fonts. Running the new 'buildafmdir' to install a new font surpresses every font above this number. Workaround: Re-install the 'old buildafmdir' from NS2.1 at /usr/bin/buildafmdir and everything should be fine!
(thanks to: Rob Parkhill and d'Art Computers/Germany d'art)
[ed: and my thanks to Borris Balzer for sending this to me]