I’m having the same problem. I created a folder on my phone and synced using DriveSync and Google Drive. I installed and tested the Obsidian app, creating a new vault on my phone and it worked fine. But as soon as I “opened folder as vault” using my synced folder, the app crashed.
Even worse, the first few times I tried, I had to uninstall the app as it wouldn’t launch without crashing. After reinstalling, I could keep it working as long as it used my “dummy” local vault, but as soon as I pointed it back at the synced folder, it crashed again.
Now, I can’t even get the app to launch after a fresh install.
I assume that there must be a corrupt file somewhere that causes the Obsidian mobile app to crash.
Very annoying, but I guess the app is very new and this is part of the teething process.
I’d be interested to hear if you fix your issue.
Edit: After deleting all the folders that I’d used as vaults off my phone, the app is once again able to launch.
Edit: The official help available from within the mobile app (“How to set up mobile sync”) specifically mentions “Third party sync” options and links to Dropsync and Folder Sync". So surely it must work for most people?