Weird message each time I open a pdf

For a few days now, every time I open a PDF file (even if Acrobat is already open), this message appears, leading to a 404 page [is says: "Acrobat shut down unexpectedly. Click on «more information» for possible solutions"] The program is working, it's updated, I've tried repairing it, uninstall/reinstall, but to no avail. What can I do? Thank you.