Resolved
Resolved in: n/a
Overview of the issue
A PHP warning or a notice is shown on the screen or added to the log, after updating only some but not all of the installed WPML-related plugins (WPML and its add-on plugins).
When WPML is installed along with other add-ons, such as Translation Management and String Translation, not updating all the add-ons may produce some PHP warnings and notices. These warning and notices disappear once all WPML add-on plugins are updated.
Workaround
Updating all active plugins solves the issue.