"Your session has ended, please reconnect." Error on login

[N-Upchurch]

Describe the bug

When logging in via SSO or password auth, I receive the following error and I am unable to log in: "Your session has ended, please reconnect."

Context

• Hardware: *VPS bought online
• YunoHost version: 11.2.12
• I have access to my server: Through SSH, through the webadmin
• Are you in a special context or did you perform some particular tweaking on your YunoHost instance?: no
• Using, or trying to install package version/branch: 1.107.0~ynh1
• Synapse is running on matrix.domain.tld, but matrix users are @username:domain.tld.

Steps to reproduce

• Install synapse and synapse-admin
• Make user admin per synapse post-install instructions
• Attempt to login to synapse-admin

Logs

Not sure if this is relevant:
Jun 04 17:17:14 python[1950124]: File '/etc/matrix-synapse/conf.d/app_service.yaml' is empty or doesn't parse into a key-value map. IGNORING.