Problems: Categories wont's show up and don't work properly in backoffice.

[Yumex]

Hello!

My question is:

Some themes I've installed wont let the categories show in the store.
Also in the backoffice, I can't make a new category, instead theres just a lot
of "missing picture" boxes and I can't write tags, name or anything.

I go back to the basic prestatheme and edit the categories, but when i change
back to the new one still won't show up in the category list. The product pages
exist, there is just no way to get there via the category section.

How can I solve this? I already selected yes for compability with older versions.
Did not work, is it something strange in the css?

Thank you for your time!