I've tried importing a PDF with no project open and I'm still getting the HTML placeholder.
Also tried importing a PDF into every kind of template. I'm baffled that this only affects me.

Some more findings, this time regarding PDF export.
If I make a blank drawing with a basic shape and try File > Export... I have no option in the list to export as PDF. If I go File > Export PDF... I can fill in a file name, but Save as type is an empty list. Clicking Export does nothing. Clicking Settings... does nothing either. Clicking Cancel after having clicked Export prompts an error: "Internal program error 5107.w.fi.gs"
I found another thread with this error message where running the program as admin fixed the problem. I tried that once again, but it didn't work for me. I get the same error message.