What you describe sounds like an issue that has been discussed on the general sketchup forum: Sierra added a new security "feature" to prevent a particular kind of hijack attack. See, for example, this topic:https://forums.sketchup.com/t/send-to-l ... 7/37181/10
Unless a newly installed app is adequately signed, Sierra runs it by first copying the code into a separate random folder it creates on the disk. This prevents the hijack, but it also makes SU unable to find LayOut. The latest release of SU 2017 has been adequately signed to correct this problem, but older versions have not because Trimble no longer supports them. So, when you reinstalled SU 2015 you likely tripped this feature. In the post linked above, Barry gives one way to fix it.