The cause and solution of "Some items could not be decrypted error"

In my case the error happened because several links in my Joplin Welcome notebooks had become corrupted. Deleting the bad link seemed to fix the error.
To find out what specific item is causing the issue in your case, copy the resource id provided in the error details window. On Windows, you must use the keyboard shortcut Ctrl + C to copy the code.
Now enter this code into your Joplin search bar. The relevant resource will be highlighted.
I observed that the error initially did not go away on deleting several of the troublesome links. It disappeared all of a sudden when I deleted one particular link. So, you may not have to remove and renter all of the resources. You might be able to get away with using Cut to remove items temporarily to see if they make the error go away and performing this test for all resources giving this error. Cannot test this further as my error has disappeared, so you might have to delete all bad resources, maybe this will be different for each case.