problems with Category post permalinks

Hi
Am having problems with the Permalinks to my posts that are listed under the Categories in the top horizontal toolbar. I’ve been using the Carrington theme. It’s been fine up to now , but recently noticed that the links to the posts are not working. You get a list of posts under that category, but when you click on the posts it only seems to momentarily go to the right post page. Then this vanishes and you get this appearing in the browser:
“https://mydomain name/”category”/maincategoryname/childcategory name-main categoryname/”
and it goes round and round looking for a page that doesn’t appear.

I don’t know if anyone else has had a similar problem but would appreciate any suggestions.