Maximum Allowed On Selection Box?

I have a script that has worked flawlessly for a while. It allows a user to select from a listbox - with highlighted lines already of their settings, multiple lines (CTRL-SHFT) and the page updates the database properly.

Now, for some reason, I have a user that when we load their settings, all is fine. If I submit the form, immediately, with no new additional selections, all works fine. But, if I add even one new selection on the list box by holding CTRL-SHFT, the submit button no longer works at all.

This user does have a long list of selections already, I am just wondering if I may have hit a max some how?