Meaning of blue circle?

Just encountered a blue circle und I have no idea what it means. It seems to effect the compatibility of the masters though…

I believe this is indicating that there is both a corner component on that node as well as a cap component across the two top nodes. That might cause issues. Click the circle to inspect/delete it or delete the cap to only see the corner.

It was two components on top of each other – thanks, Florian!