I know where exactly the crash is happening. But it does not make sense to me.
It crashes when it tries to adjust the font size on the client. A client has a size modifier stored in the settings. This modifier is by default 1.0 which means that the client always uses the same lyrics size as the host. The modifier is automatically changed when you use the pinch gesture in the client. The default value has been automatically set when updating the app to version 3.4.0 or running the app for the first time.
Somehow the default value on your client is not correctly initialized. The setting does exist but it does not contain a valid number as far as I can tell. I have absolutely no idea how that happened.
What should get you going again is this: Create a full backup on the client. Store the backup somewhere on the computer or in iCloud. Then delete and reinstall the app. Now start it. It should automatically create all default settings. Restore the backup. Now it should properly connect to a host.