Yea, crashing for me on startup as well. Was using beta4 earlier today just fine, installed beta 5 and, well, Crashy McCrashface showed up.
Created a new User account and it WORKS FINE there. Back in my standard account, completely removed app and affinitydesignerbeta files from Library (using AppCleaner). Tried reinstalling several times here as well. Also tried holding CTRL at launch to clear user data, no dice.
Couldn't see where to attach so here's my crash log:
https://www.dropbox.com/s/m5jku6su7mbfciw/crash-report.txt?dl=0