I use Mac Outlook 2016 and started seeing this each time I close out of Outlook. I am not able to get rid of the error. I realize it is saying something is wrong with the Custom Dictionary located in my account. I have done a few Google searches, but not finding a way to get rid of this error.
Note that I am able to add words to Word, but Outlook does not given an option. In fact, in Outlook, the "add" is greyed out and unable to do anything.
I had the identical issue. I just chalked it up to being on the fast insider update track. There haven't been any updates this week, which I assumed might correct the issue. I will try the fix you posted. -Jim