I have had no problem with my financial websites, but there are a lot of new security features in Safari 11. I have found that many websites have features that do not work when content blockers are enabled, but that is configurable on a website-by-website basis so you might try this. go to the website but before entering any data on the menu bar go to Safari > Settings for this website and in the drop down that appears on the Safari window un-check "Enable content blockers". That action will force the page to reload and then hopefully it will work correctly.

NOTES:
  1. you may see a small icon appear at the right end of the UserID field. Click on that and you should see a list of entries including one labeled "for this site". Click on that and the subsequent fields will be filled in.
  2. You can add an icon to the Safari 11 tool bar for Website settings in View > Customize Toolbar.
  3. The default website settings can be set in Safari > Preferences > Websites

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein