An inbound IDOC was failing due to the error [described in the title]. Some research resulted in the following results:
TR: SM12 displayed a list of locked elements in the system.
TR: SM04 displayed a list of logged in users in the local system.
TR: SM08 displayed a list of logged in users in all the app-servers [not just the current system].
Unfortunately in my case non of the mentioned transactions above helped me narrow down where the user was locking the system – in the end I’ve requested for the user to log back in and kill his session with a TR: /nex.