I always used a nick containing an "ü" on one network. However, since the update, I receive the Illegal characters error and it won't let me use that nick anymore.

It might be caused by the UTF-8 encoding. Is there any way to disable it for the nicks?

In short: no.

What you can try as a workaround at the moment is: /RAW -n NICK yournickhere

This sends the command to the server without encoding it.

sadly, this doesn't work either

I tried this and it worked for me. You might have to $utfencode the nickname if it was copied as unicode.

However you should know using such a nickname will make it impossible for you to perform any actions on yourself as a target, and possibly other problems too. You can read about why this is no longer supported here