Please try to run this terminal command: (run while BTT is quit completely)
defaults write com.hegenberg.BetterTouchTool BTTDisableSecureInputLookup YES
I think it should fix the issue for now (however this command will only work with >= 2.530 ). It looks like this is the same issue as here: macOS 10.13.4 Freezing with BetterTouchTool Usage
Thus I'm closing this topic.