Well, this bug might have different causes and therefore might require different solutions/procedures.
First it appears that if I activate the highlight function on the menu bar (i.e. "Hervorheben" in the German Version)
then the shortcuts don't work anymore.
So in my hands it works as follows:
- the menu button "Highlight/Hervorheben" should not be activated (i.e. it should not be black/grey)
- then I go to the menu button "Highlight/Hervorheben" and select a color (eg. "Yellow/Gelb") without having labeled any text within the PDF-document (that means that you would not highlight an already marked text within the PDF-document, but that you would be highlighting any text thereafter using your cursor)
- then I inactivate the menu button "Highlight/Hervorheben" by directly clicking on it (i.e. it should no longer be black/grey)
- then the shortcuts for highlighting or underlining of the text should work again - at least in my hands
- I hope that this will work for you - if not probably fool around with the sequence of events as I did and got lucky in the end
- I actually assumed for a long time that once I highlighted and/or underlined the text in a PDF-document and the short cuts stopped working that this could not be reversed
- but I was amazed that sometimes I could use the shortcuts again but couldn't figure out why
- so today by fooling around with the sequence of events I finally got lucky
- reading a lot of manuscripts, publications and other PDF-documents as I do in my job it made really furious that a simple program such as preview all of a sudden didn't work anymore as it did before
Using Macs since 1993 I can only say that Macintosh is no longer what it once used to be - a user friendly revelation in computer technology. The newer systems softwares like macOS Sierra are basically an improvement for the worse. My previous Macbook pro (the one with a DVD drive) with the systems software before El Capitan was much more reliable, faster and much more professional to use. My new Macbook pro is not a real advancement - it simply wastes my time by its shortcomings. At a certain point I even entertained the notion to switch to a PC again - alas, I have to many thousands of aliases that would be too much work to reestablish those a PC. So I have to stick with Mac - which is no fun anymore.