Well, my issue seems to be resolved, but in a totally other and weird way....!?!
I am not sure how to explain it, but I will give it a try.

In the default folder for Outlook-datafiles I assumed that only my main default datafile was stored.... nothing strange about that.
Only somehow I also had 4 other Outlook-datafiles with the names of the 4 email addresses I have.... but they were EMPTY datafiles (see attachment #1).

I added one of them in my Outlook client and saw that it was empty, only the default folders, like when you start from scratch, were present.
After I did that I (still) got the 'error'-message. For the record, after starting-up Outlook it seems that at least some of the other datafiles are activated?? (see attachment #2).

When I removed the extra datafile from the Outlook client and moved the other datafiles to a 'Backup'-folder I didn't encounter the 'error'-message anymore (see attachment #3).

I don't know how those empty datafiles appeared in that folder, I don't remember ever creating them, let alone in the last 3-4 months.
I also don't believe that they appeared by magic or by some weird error caused by Outlook?
That part remains a mystery for now, but at least I am not bothered anymore by the annoying 'error'-message.

Thanks for trying to help me and giving me more insight in Outlook, very much appreciated! My issue is SOLVED, thanks.

Attached Images

	Outlook2016 datafiles - closed.jpg (76.2 KB, 5 views)
	Outlook2016 datafiles - opened.jpg (81.3 KB, 5 views)
	Outlook2016 datafiles - only main opened.jpg (59.3 KB, 5 views)