If you use iCloud sync that might be a problem. I would say you shoul check if the .obsidian folder and it’s content is visible in your files app.
BUT to my horror I realized that dot-prefix folders are not visible on iOS.
I would suggest you backup (copy) your vault-folder on the mac or online and then remove all content from it. Check the files app on iOS that everything is gone on your device and start the Obsidian app on your phone - maybe it will not block the startup when seeing an empty vault.
if you are willing to install the app WorkingCopy, it has a nice feature called synced folder.
Adding your vault folder to a synced folder in working copy makes the .obsidian accessible for you.
(Synced folders may be a payed feature…)