There are more people who have 1.23 than people who have 1.24. Therefore, in order to reach the most people, i'd suggest hosting a 1.23 server.

Also, in the regestry using any editor, you might want to do this:

Go to HKEY_LOCAL_MACHINE
Go to Software
Go to Epic Megagames
Go to Jazz Jackrabbit 2 (Jazz Jackrabbit 2 Secret files if you're hosting a 1.24 server)
Go to List Servers
Set string values for "000" and "001" to "jazz.nimrod-online.com" (Disregarding quote marks, of course.)

There. That works for me whenever I want to host a server, and I can join too, not like before, which is quite weird. Perhaps it contacts the server listed on "001" for hosting?