After updating the DigitalOcean DNS extension in Plesk to version 1.3.9, opening it or doing authorization in it both via OAuth2 or API token fails:
Call to a member function get HeaderLine() on null
Bug in DigitalOcean DNS extension EXTPLESK-4579 which is fixed in version 1.3.10 of the extension.
(Only on Plesk 18.0.52) Apply the fix from the article https://support.plesk.com/hc/en-us/articles/14166304366999.
Update the extension DigitalOcean DNS to version 1.3.10 or newer.