By the sounds of it, it doesn't like a process that comes after the tapped hole. Has the surface over the hole been cut or anything like that, I'm just thinking it might not be able to figure out where the hole starts or something, therefore can't do it. You could try changing it to left handed and see if that will allow any mods, then change it back?
I'd be filling it in and doing it again, not ideal but something after the tapped hole is, I believe, causing the problem.
Other than that, it's definately a internal thread? I've accidently put a external thread in a hole, and changing it never worked, then I realised what had happened and started it again.
*edit*
I've just done a test. if you select the part, right click the thread in the history, and select 'replace solid' that will fill in the hole without adding more bits. will this work for you? Then just put the hole in again?
Edited by user Saturday, January 27, 2018 2:41:32 AM(UTC)
| Reason: more information added