As I recall but have not verified, that issue can affect some sites upgraded from 4.x versions which did perform a hard delete of user accounts if those deletions were done (even through the user admin area) prior to the 5.x upgrade and if the deleted users had been the last ones to create or edit HTML content.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner

I believe you still have rogue entries in your database after the upgrade.
You need to look in the HMTLTextUsers and the Users table and ascertain whether you've got unmatched HTMLTextUsers entries.
If you do you need to delete them via SQL
We suggest you either set all those unmatched users to UserID=-1 (negative 1) OR
create a fake placeholder user and link your rogue users to the fake account

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Ken Grierson
DotNetNuke Corporation
Test Lead

 
Seems a little involved... I will see if the 5.6.2 update fixes this issue when released. I am in no rush yet. I will wait until the final RC is released. Thanks for the tip.