2770865 - Neo application startup fails: Cannot generate OAuth 2.0 SAML Bearer Assertion because there is no user logged in the current thread

An SAP BTP Neo application startup fails with this error:

#ERROR#org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/com.<software component>]##anonymous#
#Exception sending context initialized event to listener instance of class com.<software component>.service.AppServletContextListenerjava.lang.IllegalArgumentException: Cannot generate OAuth 2.0 SAML Bearer Assertion because there is no user logged in the current thread.
at com.sap.core.connectivity.apiext.impl.authentication.assertion.AbstractAssertionProvider.getCurrentUserId(AbstractAssertionProvider.java:52)
at com.sap.core.connectivity.apiext.impl.authentication.assertion.AbstractAssertionProvider.getAssertionToken(AbstractAssertionProvider.java:28)
at com.sap.core.connectivity.apiext.impl.authentication.AuthenticationHeaderProviderImpl.getOAuth2SAMLBearerAssertionHeaders(AuthenticationHeaderProviderImpl.java:74)
at com.<software component>.connectivity.http.HTTPConnector.getAuthenticationHeaders(HTTPConnector.java:123)
at com.<software component>.connectivity.http.HTTPConnector.injectAuthenticationHeaders(HTTPConnector.java:78)
at com.<software component>.connectivity.http.HTTPConnector.executeGET(HTTPConnector.java:45)
...

SAP BTP  Neo Environment

OAuth SAML Bearer Assertion Authentication, System User, technical user, admin user, common user, communication user , KBA , BC-CP-DEST-NEO , Destination service for Neo environment , Problem