DigitalOcean Droplet (Ubuntu 18.04) Password Reset Loop

Question

I'm relatively new to DigitalOcean droplets, but I have had a Ubuntu 18.04 droplet running a Nextcloud instance for a couple weeks before I started running into problems. I'm not 100% sure where the problem lies, and my troubleshooting hasn't gotten me very far.

I misconfigured one of the Nextcloud client machines, and it pushed more files than there was room for. My Droplet is at 100% usage, and the client machines cannot connect to it. It is available for SSH.

When I SSH into the machine or use the DO console, I'm immediately asked to change the root password. Once the change is successful, it immediately closes the connection. Connecting again, it asks again for a password change - I'm stuck in a loop.

SSH -vv output:

Changing password for root.
(current) UNIX password: 
Enter new UNIX password: 
Retype new UNIX password: 
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: client_input_channel_req: channel 0 rtype [e-mail] reply 0
debug2: channel 0: rcvd eow
debug2: channel 0: close_read
debug2: channel 0: input open -> closed
debug2: channel 0: rcvd eof
debug2: channel 0: output open -> drain
debug2: channel 0: obuf empty
debug2: channel 0: close_write
debug2: channel 0: output drain -> closed
debug2: channel 0: rcvd close
debug2: channel 0: almost dead
debug2: channel 0: gc: notify user
debug2: channel 0: gc: user detached
debug2: channel 0: send close
debug2: channel 0: is dead
debug2: channel 0: garbage collecting
debug1: channel 0: free: client-session, nchannels 1
Connection to 159.XX.XXX.XX closed.
Transferred: sent 3772, received 5100 bytes, in 10.9 seconds
Bytes per second: sent 346.6, received 468.6
debug1: Exit status 255