Bug in course display – users see blank white page instead of content

I have an active course, and some of my users report that when they try to open the course, they only see a blank white page.

The course appears in their list, but when they click into it — the content doesn't load at all.

From my side (as admin), everything looks fine — all lessons and sections are visible.
I double-checked permissions, and everything seems correctly configured.

What could be causing this?
How can I urgently fix it?