After upgrading macOS to Sierra 10.12.5, texshop stopped working: it could open tex files but not typeset. The error message was similar to this:
Library/TeX/texbin /Library/TeX/texbin /Library/TeX/texbin >/Library/TeX/texbin/pdflatex does not exist. TeXShop is a front end for TeX, but >you also need a TeX distribution. Perhaps such a distribution was not installed >or was removed during a system upgrade. If so, go to http://tug.org/MacTeX and >follow the instructions to (re)install MacTeX. A less likely possibility is that >a tool path is incorrectly configured in TeXShop preferences. This can happen if >you are using the macports or fink distributions.
(I can't get the original error message because things got worse...)
After reading various posts, including one here, which had the above quote, I installed the latest mactex2017. Now things are worse: texshop won't open at all.
Error message long. Excerpts:
Path: /Applications/TeX/TeXShop.app/Contents/MacOS/TeXShop Identifier: TeXShop Version: 3.77 (3.77) ... Dyld Error Message: Library not loaded: @rpath/Sparkle.framework/Versions/A/Sparkle Referenced from: /Applications/TeX/TeXShop.app/Contents/MacOS/TeXShop Reason: Incompatible library version: TeXShop requires version 1.6.0 or later, > but Sparkle provides version 1.5.0
Hope that's relevant.