I read the same thing and we have always put our clients on Office 2013 for this reason. MS did an update that forced some computers to upgrade to Office 2016 without asking thru automatic updates. If you look at some previous replies there is a person who has always been on 2016 and it was fine until the latest office update. The biggest issue I see is how long are you going to be able to get the downgrade to Office 2013. This really needs to be addressed.
I have had many calls regarding that issue , every time I will try something new
one of the cases ( the one that worked with me ) the were already and have always been on office 2016
uninstalled all updates happened in 2017 still same issue
disabled updates in Microsoft word settings
restored windows ( using system restore ) back 3 weeks to before any of these updates happened , after the reboot I was able to use word 2016 to merge letter .
SD support has given me a fix.
Login as the administrator on the local computer.
Merge letters should be working now.