I am having this exact issue as well. It keeps looping back to downloading the latest software update, but even after uninstalling it and trying different pathways, I can't even get past logging in to download the product (MODO Bass 2) that I just bought. I emailed tech support so hopefully they get back within a few days, otherwise I'm done with IK Multimedia. I used to love using Amplitube but I remember a similar issue way back when... I hope they reach out for a fix.
Running an M2 MacBook, with macOS Sequoia 15.1.1. Oh, and PLENTY OF SPACE for this plugin lol...