When trying to trigger a SAML logout using the "Disconnect from [IDP]" link the following error gets displayed in the browser:

ERROR : An error occurred on 2021-12-14 at 21:00:21

The log file contains the following error:

2021-12-14 20:33:27.991 ===== TRACE ===== DISCONNECTED USER '[REDACTED]'
2021-12-14 20:33:28.012 ** ERROR ** [V9.3.5] ERROR **
2021-12-14 20:33:28.012 ** ERROR ** [V9.3.5] on file '/opt/project/sso/projeqtor/index.php' at line (43)
2021-12-14 20:33:28.012 ** ERROR ***** [V9.3.5] cause = Undefined variable: nameIdSPNameQualifier

Initializing the 'nameIdSPNameQualifier' variable before using it in sso/projeqtor/index.php solves the problem.