Is this localized to one file? Being symptomatic on two stations it is potentially just a file in need of round trip export to idml, open, save as and go.
That certainly wouldn't hurt, but it seems more symptomatic of corrupt prefs, or possibly user error (sorry we don't really have enough info to rule this out -- it could be that you need to include a section prefix). You say they can't enter anything. Does that mean typing doesn't work, or that you can type, but you don't get expected results?
Both of the operators are seasoned professionals with over 20 years of publishing experience. I'm inclined to omit user error.
The field in the Export dialog does not allow any text entry. All keypresses are ignored. If the dialog is closed the users can edit text in the document, switch to another program and enter text without any problems. It's just this text field that won't allow input.
Our kludgy workaround was to export all pages and then delete all the pages we didn't need so this is not a "show stopper" but more of an odditiy.
I deleted the cache files for one user and I think that may have helped. It also appears that these problems are restricted to a group of documents from one client. I'll ask them to export to INX and then re-open the INX files and see if that helps out.
Prior to your last paragraph, I would say this problem is consistent with corrupt preferences. A corrupt document should not be able to cause this problem (but "should not" is not the same as "can not."). Definitely go for INX.
Can they enter other figures in the PDF Export, such as, say, compression ppi thresholds?