Clicking on the "expand" widgets [+] on Wikipedia for numerical (number) only categories gives: "Problem loading data. Please wait a moment and try again" error messages.
To reproduce this problem go to this page, for example...
...and click on the "expand" widgets [+] for all the 100, 101, 102...to 109 categories. This gives "Problem loading data. Please wait a moment and try again" error messages.
Then try for the "100s births" category on that page, this will work properly, as expected.
It seems that category names made up of 'numbers only' give this error, but if one or more letters are added to the category name, the error does not show.