[baruch]

You have either brute force with more characters or if you have a file in there with a known content there is a known-plaintext attack on zip encryption. I've used it once and it worked nicely but it's rather tricky to find another file with the exact same content for this.

[lampzzy]

wow! this is a good one! The Zip contains not only the logs but all the files that were part of the mIRC application (executable, config, etc). Maybe I'll try to find some old config files and explore this option.

[baruch]

The utility I used is PkCrack: https://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html