I was able to set up the normal s2e-functionality (and verified it by external servers/clients), and also was able to bring an own program (and run it via a telnet shell) to the xport pro, which outputs to the serial line. I removed the respawning ttyS0::askfirst:-/bin/sh in etc/inittab for both purposes to have a free serial device.

I am not able to set up the s2e-ssl functionality. In the menuconfig / Lantronix' Applications I added to build s2e-ssl.
After compiling/loading, I do not get the additional entry for ssl in the browser interface as shown on page 57 of the Linux-SDK_UG, nor does the Protocol-dropdown-list in the Tunnel Setup offer the SSL option in addition to TCP and UDP.

Do I have to include anything else (a library or something)?

Any help is appreciated!