Help! Blog Category Link is broken

Hi, A broken link report has identified that one of the blog category links, that should go to a collection page is broken.

Other categories are working fine, but the Portfolio - Original Paintings + Prints category link goes to a 404 error page, rather than a collection page.

Do i need to delete this category and add it again to all the posts? or is there something else going on?

thanks for your help

Category Personal also seems to have a problem. but category Art Update works fine out of the box.