Welcome back WriteAndFight
This sounds rather worrying. From the question alone, I would wonder if something is failing in your BPs. Though the cause can be many things and the remedy is also varied. As suggested by Humanasset, you could always go line by line and breakpoint or print-string your way through this.
The end user ended up making a new blueprint and that resolved the error. The same result occurred for Ianu_Gor. I would wonder if this would also work for you.
I would like to hear what solutions worked for you. I wish you the best.