OK, so it's not browser-specific; that's helpful.
I can't think of anything else to suggest - because that situation (no other plugins + default theme) shouldn't be having this problem. (I can't reproduce it; if it was normal, then we'd get lots of reports, since UD is downloaded about 1,000 times a day). Would you be willing to email me your login details so that I can take a closer look? (contact@updraftplus.com). Failing that, I can't think of anything else to suggest.
David