All Functionality restored now! Here’s how:
Re-phrased problem description
OH Items which where previously linked to a virtual actuator (=variable stored in the Software from the SmartHome company) wouldn’t change state on using sendCommand().
Origin of the Problem
The virtual actuators were deleted long ago. They continued to live on as virtual variables inside OH only. Apparently, the links have not been revoked. After an addition in the OH 2.4 software the items tried to communicate with not existing actuators, and the autoupdate feature prevented the items from changing state.
Solution
Following up on @rossko57 suggestion (thanks so much!)
I installed the “REST documentation” via PaperUI → Add-ons → Misc. On the OH Welcome Screen one can access the Add-on. When clicking on ‘links’ one can ‘GET’ information over all items just by clicking the “Try it out!” button and then search for the itemName using one’s browser’s search functionality.
One should find information similar to the following
{
"channelUID": "smarthome:VariableActuator:{NumberHere}:{AnotherNumberHere}:switch",
"configuration": {},
"itemName": "zBett"
},
Using the channelUID and itemName in the ‘DELETE’ part of the link-section of the Add-on one can unlink the undesired (ghostly) channels.
That’s what did the magic for me. Items change state again on sendCommand(), changed Items trigger rules and will show up in the openHAB-cloud. Maybe, @Kim_Andersen, after understanding my own problem better 
, this might help you after all(?).