I get a "mIRC has encountered a problem and needs to close. We are sorry for the inconvenience." error while conecting.

The last lines i see are:

End of /MOTD command.
-
-Sterling.VA.US.UnderNet.org- on 1 ca 1(2) ft 10(10)
-

or any server in the place of sterling...

Try typing /remote off before you connect. Then connect and see if you still have problems. If it works fine after using that command, you have a problem with a script.

-genius_at_work

Also, make sure you're not using a DLL, normal scripts can cause errors but if you load a DLL its not like a script and if it does something that is not able to be handled, buffer overflow, access violations etc it can cause the client to crash.

Do as genius_at_work says, it can stop the problem in some cases. If it does work, then check to make sure you do not have any DLLs being loaded, if so, unload the script that loads it and then try.