Thanks, Sebastian and Matthias.
Well, I found that we had created a custom module for a contact form and it's actually named MailForm. The list of installed modules on the DNN GUI doesn't show the module code name, so it wasn't apparent unless one went to the \App_Code or \Desktop Module folder on the Web site and take a look around.
Something adding to the confusion is that one would expect an "error Installing Package File MailForm_01.00.00_Install" (or any other package) on the upgrade log only for modules being updated by the DNN upgrade routine. So, this error is a bit misleading.
Why would the DNN upgrade process signal an error on updating a module not provided and not being able to be updated by DNN?
Is it that the upgrade proc checks if 3rd party modules can run correctly under the upgraded version of DNN?
(I haven't properly tested our form since we need to change the SMTP server as we have changed providers. Our hosting provider has different SMTP servers for apps running on the host than those running remotely, so I have to ensure we have the right settings. I'll report back on that.)
Perhaps the Upgrade proc needs to alter all module packages, even if 3rd party?
Thanks for the info.