DNS pool isn't an error. It's just telling you that it's trying the next server in a pool (list) of servers.
Try changing your nick to something that starts with a letter rather than the underscore character. As your post shows, the network doesn't allow that type of nick. See if that helps at all.
You also said UPP works when connecting. Because UPP is just a script, if it works, then mIRC without scripts should work assuming you have the same settings in mIRC (UPP may change the default mIRC settings to something else). Another script could fail and cause the freezing and if that's the case, you'll need to probably check with the authors of the script that is freezing. If UPP is working and mIRC without scripts appears to be freezing, you can try installing UPP and the press Alt-R and use File > Unload to remove all scripts in the Remotes and Aliases tabs. That will leave you with the same settings in mIRC without any scripts (UPP) loaded. So you'll basically have mIRC without scripts, but with a specific set of settings. You can then test it to see if that works. If so, then it's a setting in mIRC that you need to change.
One option you might want to look into is to disable UPnP in mIRC's options. It can cause freezing if the computer or router doesn't support it very well. See what happens when you disable it. My guess is it's disabled in UPP, but it's normally enabled in a clean install of mIRC.
