Col 285 Sector -J c9-34, host to around 40 objects including a ringed ELW, but only has one single build slot. I suspect the colonization system is seeing all the objects orbiting a trinary and deciding none are suitable?
The system is now missing the primary port slot, so it is not yet fully resolved. I have already stumbled across similar ones which cannot be claimed either.
Maybe you should have boldly claimed the system when it was still a single slot with the flag symbol? Or had you even tried that?