I ended up finding a thread about what I believe is the same or a similar problem, and has to do with the Wilson theme breaking with the 1.26 update:
https://wordpress.org/support/topic/category-and-tag-pages-are-now-all-blank/
There is a fix, but since I’m not a premium user it’s not something I can implement myself (since I can’t edit theme files, just css); I’m hoping it’s not too much to ask to have the theme rolled back to 1.24 (at least until it’s fixed) or the fix implemented?
I seem to have bad luck with picking themes that end up breaking.