I don't know if this is the correct way to fix this, but..
I edited .config/aprsd/aprsd.conf
changed callsign under DEFAULT settings to owner_callsign, pressed the Save Configuration button and tried webchat again. It worked.
# CRITICAL - <No description provided>
[DEFAULT]
#
# From aprsd.conf
#
# Callsign to use for messages sent by APRSD (string value)
owner_callsign = YOUR_CALLSIGN_HERE