The defaults (Tools > Reset Configuration) were changed. You will have to make a custom change to your preferences: Edit > Preferences > Tracks Behaviors > Solo Button: Simple
Sounds like this problem: 3.4.1 Crashes trying to open 3.3.3 .aup3 project · Issue #5568 · audacity/audacity · GitHub
So probably you just want to revert to 3.3.3: Old Audacity versions download