On this page:

and various others with a search box that returns users, there's an expandable set of advanced options at the bottom.

If you hover over the header text ("Search Options" in english) you get a pop-up which shows the title text: "Click to expand or collapse". However, clicking on the text does nothing. To expand/collapse the options you need to click on the arrow image at the end of the text.

This arrow image has two accessibility issues. One, it's a very small click target, which makes it hard for people to use on iPhones, iPads etc. and two, although it's the only thing inside a click-able anchor tag, the image has no alt text.

I'd suggest making the the clickable area the whole text, which solves both problems. I'd also think about changing the open/close state to a class that gets toggled, rather than directly changing an img src (which is what I assume is happening now). This gives many more options for presenting the change in state.