Installed mbstring extension, now experiencing UI issues with WP

I installed the mbstring extension, so that I could use the Relevanssi plugin. As soon as mbstring was installed, I began experiencing UI issues in WordPress (cross-browser: most current versions of Firefox, Chrome, and IE on a PC with Win 7). The list is a little miscellaneous, but includes the following kinds of issues (all a little ajax-y):
* When editing page or post, I can’t toggle between Visual and Text views.
* When editing a page or post, selecting/deselecting screen options doesn’t work: I can check the boxes, but the page doesn’t update.
* When editing a page or post, I can click the “Edit” button to change the slug, but the page doesn’t update.
* When using quickedit from “all pages” or “all posts”, if I attempt to save any changes, they will commit — but the UI shows an error as though it has not saved correctly. (I see the “Are you sure you want to do this?Please try again.” message next to the cancel button.)
* Drag & drop don’t work in Menus or in Widgets.

I’ve tried disabling all plugins, which has no effect — I believe the issue is somehow related to mbstring.

Thanks,
Jessica