The following error appears several times in NX_ROOT\log\stdlog:
web:local 16664 ERROR session.c 10397 Session id (SessionID1) for Session User1-SessionID1
Service Desk Manager 17.x
The message is more of a warning which can be ignored. The message just says that the session was already removed from the list and we are trying to remove it right now. It doesn't change the execution path in any way.
These messages have been changed to WARN instead of ERROR starting from 17.4 RU3