To determine which version of Office 2016 - in Excel, please go into menu File / Account and look at "About Excel"

If the version is "Version 1804 (Build 9226.2114 Click-to-Run)" the below workaround is applicable.

(for more information on the delivery of Office Builds by Microsoft through different channels - see 

for more information about the Microsoft Office 2016 Updates Roll Back procedure described below - see )

This is solved by upgrading Planner to the build 4232 or later.