File locked and can't be deleted online or from clients


I am a user on a hosted Nextcloud 13 instance. I’ve set up sync between two machines and can read/write files online and offline and things are synced well.

However, there is one file in one of my folders that seem to be “locked”. If I try to delete it from the web interface, I get this error as a notification:

Error deleting file "[my file]".

If I delete the file locally, my Nextcloud client logs this error:

Error transferring [my file] - server replied: Locked

I searched a bit and have seen solutions for server administrators, but I am just a user. Why is this file locked, and how do I unlock it? Thank you!