Error on Login: "TypeError: Cannot read properties of undefined (reading 'toLowerCase')"

Symptoms

• Plesk is not login to Plesk UI:

  PLESK_ERROR: TypeError: Cannot read properties of undefined (reading 'toLowerCase')

• Log from /var/log/plesk/panel.log shows:

  CONFIG_TEXT: [Action Log] Failed login attempt with login '<invalid>' from IP 203.0.113.2

Cause

The issue is caused by the Kaspersky Antivirus program installed on the PC used to login to Plesk. The request is intercepted by the antivirus program before being sent, which causes an error.

Resolution

Add Plesk URL in the exception list of the Antivirus.