Lare, I think the problem is clearly with the logic which isn't checking the activity bit or even writing to the activity bit and exchange number to the management array at all. I have found if a READ_VAR block has a failed exchange it won't complete another one until the exchange is cancelled as the documentation indicates. If it is used incorrectly from the start sometimes the only way to get it to start working correctly is to make a new instance of the READ_VAR block.
I am surprised the logic in the screenshots posted works at all.
I think Krauly should start by importing the complete working example program section for using READ_VAR with serial that I linked to, or post screenshots of the logic which shows that the missing pieces you have pointed out have been addressed, or there is no point in troubleshooting the link when the logic is incomplete