Solved. stty -a said 0 rows 0 columns. Changed that to 24 rows and 80 columns, now joe works. Just have to look for if it is possible to set these values in the terminfo file instead in the ~/.profile.
I'm trying to use Joe (4.6) from serial connected terminals with a NetBSD server. While Joe can be run perfectly on the console as well as using telnet or ssh connections, it fails when started using a serial terminal. No matter what TERM is set to (vt5120, vt320, vt220, vt100), Joe only uses line 1 of the display and is hence unusable. Any idea what this is caused by and how to get Joe to use the entire terminal, even on serial lines?