A problem has been fixed where a user can not take over a recurring task (C_PERIOD object) although the client has the rights to take over tasks and has access to the object/task.
Upgrade to the fix version below or a higher one.
Automation.Engine 12.0.7 - estimated, planned release date Mid of January 2019
Automation.Engine 12.1.4 - estimated, planned release date End of January 2019
Automation.Engine 12.2.1 - estimated, planned release date End of October 2018