I reported all fixed a little too soon but in the end all has worked out.
All sites except #WC updated okay from 2.1.0 to 2.2.1, dashboard messages displaying as expected. However #WC displayed an error message at the top of each older message (but not the new message added today):
Notice: Undefined offset: 0 in /home/wvccnet/public_html/wp-content/plugins/custom-dashboard-messages/custom-dashboard.php on line 148
All websites are the same theme and mostly the same plugins, everything up to date. Puzzled why only one site had this problem yet thankful only one site.
I do not use dashboard message titles on any of the sites. The large font and line spacing take up too much space for my purpose. Wondering if no title could be an issue? I tried an edit-update with no text changes, no fix. I edited to add a title such as X. Error message still there, showing above title X. Edited again to remove title X and finally the error message disappeared on that one record.
I added a dummy title, then removed the title, for 25 records for this site.
Happy only one site required edits.
Thought you should know.