Each time I try to start Signal. It immediately crashes.

15.2 has a major problem with Thunderbolt devices. Dozens of reports on the discussion forum after people upgraded from prior versions. In my case 15.1 was working flawlessly. After upgrading to 15.2 the MAC PRO (2019) won't even boot with multiple Thunderbolt devices attached. If as suggested I disconnect all those devices the MAC boots but I no longer have my Thunderbolt drives available. Someone from Apple really needs to start listening to this. We have posted feedback after feedback and I keep getting told no one else has this problem. Well 4 pages of people on the discussion forum would disagree with that.

I have two thunderbolt LG 4K monitors. After upgrading to beta 3 the startup hangs half way through the progress bar. If I unplug one monitor the system starts up normally. Once up I can plug the second monitor in and all works as expected.