i dont know why you download UdpEngine.class and UdpEngine$1.class or where you do it. i suggest to delete de install and download again.
After it is installed, you need to edit the file projectzomboid64.json (if you use a 32 bit system edit the 32.json or the batch files) and change option "zomboid.steam = 1" to equal 0 (zomboid.steam =0). also if the server hangs you may need to change the allocated ram memory to less than your systems max. that you can change in startserver64.bat or startnonsteam.64 default values is 16 gb expresses as "-Xms16g -Xmx16g" change to 4bg "-Xms4g -Xmx4g". the problem with the lua i dont remember face it, but i remember having problems with diferent weapons files version from the server and client, a fast way to solve it, is to copy the files in conflict from the server to the client, the upnp can be desactivated once the server run once and created the file servertest.ini, upnp only is for open ports on router automatic but i found it only works when the firewall is desactivated.
Also if you have the game in steam you need to change projectzomboid64.json zomboid.steam =0 to be able to connect.
one more thing, any client who not match the server version will be unable to connect.